UNIVERSITY  OF  CALIFORNIA 

DEPARTMENT  OF  CIVIL   ENGINEERING 

BERKELEY.  CALIFORNIA 


Ciifii 


UNIVERSITY  OF  CALIFORNIA 

DEPARTMENT  OF  CIVIL   ENGINEERING 

BERKELEY,  CALIFORNIA 


GENERAL  LECTURES 

ON 

ELECTRICAL  ENGINEERING 


^MQ  Qraw'3/ill  Book  &  7ne 

PUBLISHERS     OF     BOOKS     F  O  R^/ 

Coal  Age  v  Electric  Railway  Journal 
Electrical  World  v  Engineering  News-Record 
American  Machinist  v  The  Contractor 
Engineering 8 Mining  Journal  v  Power 
Metallurgical  6  Chemical  Engineering 
Electrical  Merchandising 


GENERAL  LECTURES 

ON 

ELECTRICAL  ENGINEERING 


BY 
CHARLES  PROTEUS  STEINMETZ,  A.M., PH.D. 


FIFTH  EDITION 


COMPILED   AND   EDITED  BY  ,       ,. 

JOSEPH  LERiOT  -HA.YDEN 


McGRAW-HILL  BOOK  COMPANY,  INC. 

239  WEST  39TH  STREET.     NEW  YORK 


LONDON:  HILL  PUBLISHING  CO.,  LTD. 

6  &  8  BOUVERIE  ST.,  E.  C. 

1918 


Engineering 


COPYRIGHT,  1918,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 


COPYRIGHT,  1908,  BY 
ROBSON  &  ADEE 


*:;{':>     v.'i-iH 


THE    MAPLE    PRESS    YORK    PA. 


PREFACE  TO  THE  FIFTH  EDITION 

In  the  eight  years  since  the  earlier  editions  of  "General 
Lectures  on  Electrical  Engineering"  appeared,  material 
changes  have  taken  place  in  the  electrical  industry.  Elec- 
tricity has  made  enormous  forward  strides,  until  it  now 
seems  destined  to  become  the  universal  medium  of  the 
world's  energy,  through  the  agency  of  huge  power  generating 
systems.  The  small  electric  generating  stations  and  the 
machinery  used  in  them,  are  rapidly  disappearing  before 
the  substations  of  the  unified  power  system. 

The  carbon -filament  incandescent  lamp,  after  holding 
sway  for  a  quarter  of  a  century,  has  become  of  mere  his- 
torical interest,  and  the  mazda  lamp  has  taken  not  only 
the  place  of  the  carbon-filament  incandescent  lamp, 
but  to  a  great  extent  of  the  arc  lamp  as  well,  so  that  the 
industrial  importance  of  arc  lighting  has  greatly  decreased 
as  compared  with  that  of  incandescent  lighting. 

As  a  result  of  these  and  many  other  developments, 
the  preparation  of  the  fifth  edition  required  material 
changes  and  additions  to  the  previous  text.  A  large  part 
of  the  book  has  been  entirely  rewritten,  so  that  as  it  now 
stands  it  differs  materially  from  the  previous  editions. 

CHARLES  PROTEUS  STEINMETZ. 
CAMP  MOHAWK,  VIELE'S  CREEK, 
SCHENECTADY,  N.  Y. 
September,  1917. 


381455 


PREFACE  TO  FIRST  EDITION 

The  following  lectures  on  Electrical  Engineering  are 
general  in  their  nature,  dealing  with  the  problems  of  gen- 
eration, control,  transmission,  distribution  and  utilization 
of  electric  energy;  that  is,  with  the  operation  of  electric 
systems  and  apparatus  under  normal  and  abnormal  con- 
ditions, and  with  the  design  of  such  systems ;  but  the  design 
of  apparatus  is  discussed  only  so  far  as  it  is  necessary  to 
understand  their  operation,  and  so  judge  of  their  proper 
field  of  application. 

Due  to  the  nature  of  the  subject,  and  the  limitations  of 
time  and  space,  the  treatment  had  to  be  essentially  de- 
scriptive, and  not  mathematical.  That  is,  it  comprises  a 
discussion  of  the  different  methods  of  application  of  elec- 
tric energy,  the  means  and  apparatus  available,  the  dif- 
ferent methods  of  carrying  out  the  purpose,  and  the  rela- 
tive advantages  and  disadvantages  of  the  different  methods 
and  apparatus,  which  determine  their  choice. 

It  must  be  realized,  however,  that  such  a  discussion  can  be 
general  only,  and  that  there  are,  and  always  will  be,  cases  in 
which,  in  meeting  special  conditions,  conclusions  regarding 
systems  and  apparatus  may  be  reached,  differing  from  those 
which  good  judgment  would  dictate  under  general  and 
average  conditions.  Thus,  for  instance,  while  certain 
transformer  connections  are  unsafe  and  should  in  general 
be  avoided,  in  special  cases  it  may  be  found  that  the  danger 
incidental  to  their  use  is  so  remote  as  to  be  overbalanced 
by  some  advantages  which  they  may  offer  in  the  special 
case,  and  their  use  would  thus  be  justified  in  this  case.  That 
is,  in  the  application  of  general  conclusions  to  special  cases, 
judgment  must  be  exerted  to  determine,  whether,  and  how 
far,  they  may  have  to  be  modified.  Some  such  considera- 
tions are  indicated  in  the  lectures,  others  must  be  left  to  the 
judgment  of  the  engineer. 


viii  PREFACE  TO  FIRST  EDITION 

The  lectures  have  been  collected  and  carefully  edited  by 
my  assistant  Mr.  J.  L.  R.  Hayden,  and  great  thanks  are  due 
to  the  publishers,  Messrs.  Robson  &  Adee,  for  the  very 
creditable  and  satisfactory  form  in  which  they  have  pro- 
duced the  book.  CHARLES  P.  STEINMETZ. 

SCHENECTADY,  N.  Y. 

Sept.  5,  1908. 


CONTENTS 

PAGE 

PREFACE    v 

First  Lecture — General  Review i 

Second  Lecture — General  Distribution 14 

Third  Lecture — Light  and  Power  Distribution 27 

Fourth  Lecture — Load  Factor  and  Cost  of  Power 41 

Fifth  Lecture — Long  Distance  Transmission 51 

Sixth  Lecture — Higher  Harmonics  of  the  Generator  Wave 68 

Seventh  Lecture — High  Frequency  Oscillations,  Surges  and  Impulses.  79 

Eighth  Lecture — Generation 88 

Ninth  Lecture — Hunting  of  Synchronous  Machines 100 

Tenth  Lecture — Regulation  and  Control 109 

Eleventh  Lecture — Lightning  Protection 116 

Twelfth  Lecture — Electric  Railway 129 

Thirteenth  Lecture — Electric  Railway  Motor  Characteristics     .    .    .    .143 

Fourteenth  Lecture — Alternating-Current  Railway  Motors 152 

Fifteenth  Lecture — Electrochemistry 172 

Sixteenth  Lecture — The  Incandescent  Lamp 1 80 

Seventeenth  Lecture — Arc  Lighting 198 

Eighteenth  Lecture — Modern  Power  Generation  and  Distribution    .    .210 

APPENDIX  I — Effect  of  Electrical  Engineering  on  Modern  Civilization.  225 

APPENDIX  II — Overhead  Line  Tables 241 


GENERAL  LECTURES 

ON 

ELECTRICAL  ENGINEERING 

FIRST  LECTURE 
GENERAL  REVIEW 

In  its  economical  application,  electric  power  passes 
through  the  successive  steps:  generation,  transmission, 
conversion,  distribution  and  utilization.  The  require- 
ments regarding  the  character  of  the  electric  power  im- 
posed by  the  successive  steps,  are  generally  different, 
frequently  contradictory,  and  the  design  of  an  electric 
system  is  therefore  a  compromise.  For  instance,  electric 
power  can  for  most  purposes  be  used  only  at  low  voltage, 
no  to  600  volts,  while  economical  transmission  requires 
the  use  of  as  high  voltage  as  possible.  For  many  purposes, 
as  electrolytic  work,  direct  current  is  necessary ;  for  others, 
as  railroading,  preferable;  while  for  transmission,  alternat- 
ing current  is  preferable,  due  to  the  great  difficulty  of 
generating  and  converting  high-voltage  direct  current. 
In  the  design  of  any  of  the  steps  through  which  electric 
power  passes,  the  requirements  of  all  the  other  steps  so 
must  be  taken  into  consideration.  Of  the  greatest  im- 
portance in  this  respect  is  the  use  to  which  electric  power 
is  put,  since  it  is  the  ultimate  purpose  for  which  it  is  gen- 
erated and  transmitted;  next  in  importance  is  the  trans- 
mission, as  the  long-distance  transmission  line  usually  is 
the  most  expensive  part  of  the  system,  and  in  the  trans- 


"2:  *  -  ?GEtfERAL  LECTURES 

mission  the  limitation  is  more  severe  than  in  any  other 
step  through  which  the  electric  power  passes. 

The  main  uses  of  electric  power  are: 

General  distribution  for  lighting  and  power.  The  relative 
proportion  between  power  use  and  lighting  may  vary 
from  the  distribution  system  of  many  small  cities,  in  which 
practically  all  the  current  is  used  for  lighting,  to  a  power 
distribution  for  mills  and  factories,  with  only  a  moderate 
lighting  load  in  the  evening.  . 

The  electric  railway. 

Electrochemistry. 

For  convenience,  the  subject  will  be  discussed  under  the 
subdivisions: 

1.  General  distribution  for  lighting  and  power. 

2.  Long-distance  transmission. 

3.  Generation. 

4.  Control  and  protection. 

5.  Electric  railway. 

6.  Electrochemistry. 

7.  Lighting. 

CHARACTER  OF  ELECTRIC  POWER 

Electric  power  is  used  as— 

(a)  Alternating  current  and  direct  current. 

(b)  Constant  potential  and  constant  current. 

(c)  High  voltage  and  low  voltage. 

(a)  Alternating  current  is  used  for  transmission,  and 
for  general  distribution  with  the  exception  of  the  centers  of 
large  cities;  direct  current  is  usually  applied  for  railroad- 
ing. For  power  distribution,  both  forms  of  current  are 
used;  in  electrochemistry,  direct  current  must  be  used  for 
electrolytic  work,  while  for  electric-furnace  work  alter- 
nating current  is  preferable. 


GENERAL  REVIEW  3 

The  two  standard  frequencies  of  alternating  current  are 
60  cycles  and  25  cycles.  The  former  is  used  for  general 
distribucion  for  lighting  and  power,  the  latter  is  often 
preferred  for  conversion  to  direct  current,  for  alternating- 
current  railways,  and  for  large  powers. 

In  Europe  50  cycles  is  standard  frequency.  This 
frequency  still  survives  in  this  country  in  Southern  Cali- 
fornia, where  it  was  introduced  before  60  cycles  was 
standard. 

The  frequencies  of  125  to  140  cycles,  which  were  stand- 
ard in  the  very  early  days,  30  years  ago,  have  disappeared. 

The  frequency  of  40  cycles,  which  once  was  introduced 
as  compromise  between  60  and  25  cycles,  is  rapidly  dis- 
appearing, as  it  is  somewhat  low  for  general  distribution, 
and  higher  than  desirable  for  conversion  to  direct  current. 
It  was  largely  used  also  for  power  distribution  in  mills 
and  factories  as  the  lowest  frequency  at  which  arc  and 
incandescent  lighting  is  still  feasible;  for  the  reason  that 
4o-cycle  generators  driven  by  slow-speed  reciprocating 
engines  are  more  easily  operated  in  parallel,  due  to  the 
lower  number  of  poles.  With  the  development  of  the 
steam  turbine  as  high-speed  prime  mover,  the  conditions 
in  this  respect  have  been  reversed,  and  60  cycles  is  more 
convenient,  giving  higher  turbine  speeds:  1800  and  3600 
revolutions  respectively  with  the  four-polar  and  bipolar 
6o-cycle  machine,  against  750  and  1500  revolutions  at 
25  cycles,  and  thereby  higher  steam  economy  and  lower 
cost  of  the  turbo-alternator,  except  in  very  large  sizes. 

Sundry  odd  frequencies,  as  30  cycles,  33  cycles,  66 
cycles,  which  were  attempted  at  some  points,  especially 
in  the  early  days,  have  not  spread;  and  frequencies  below 
25  cycles,  as  15  cycles  and  8  cycles,  as  proposed  for  rail- 
roading, have  not  proved  of  sufficient  advantage  so  that 


4  GENERAL  LECTURES 

in  'general,  in  the  design  of  an  electric  system,  only  the 
two  standard  frequencies,  25  and  60  cycles,  come  into 
consideration.  At  present,  the  tendency  is  strongly  to- 
wards 60  cycles,  and  the  use  of  60  cycles  is  extending  more 
rapidly  than  that  of  25  cycles,  especially  since  fairly  good 
6o-cycle  converters  have  been  accomplished. 

(b)  Constant  current,  either  alternating  or  direct,  that  is, 
a  current  of  constant  amperage,  varying  in  voltage  with  the 
load,  is  mostly  used  for  street  lighting  by  arc  lamps  and 
by  series  incandescent  lamps;  for  all  other  purposes,  con- 
stant potential  is  employed. 

(c)  For  long-distance  transmission,  the  highest  permis- 
sible voltage  is  used;  for  primary  distribution  by  alterna- 
ting current,   2200  volts,   that  is,  voltages  between  2000 
and  2600;  for  alternating-current  secondary  distribution, 
and    direct-current    distribution,    220    to    260    volts    and 
occasionally    twice    this    voltage,    and   for   direct-current 
railroading,    550    to    600    volts,    and    occasionally    1200, 
1800  and  even  2400  and  5000  volts;  for  alternating-current 
railroading  usually  11,000  volts. 

i.  General  Distribution  for  Lighting  and  Power. — 
In  general  distribution  for  lighting  and  power,  direct  current 
and  60  cycles  alternating  current  are  available.  Twenty- 
five  cycles  alternating  current  is  not  well  suited,  since  it 
does  not  permit  arc  lighting,  and  for  incandescent  light- 
ing it  is  just  at  the  limit,  where  under  some  conditions 
and  with  some  generator  waves,  flickering  shows,  while 
with  others  it  does  not  show  appreciably. 

The  distribution  voltage  is  determined  by  the  limitation 
of  the  incandescent  lamp,  or  about  no  volts.  One  hundred 
and  ten  volts  is  too  low  to  distribute  with  good  regu- 
lation, that  is,  with  negligible  voltage  drop,  any  appreciable 
amount  of  power,  and  so  practically  always  twice  that 


GENERAL  REVIEW  5 

voltage  is  employed  in  the  distribution,  by  using  a  three- 
wire  system,  with  no  volts  between  outside  and  neutral, 
and  220  volts  between  the  outside  conductors,  as  shown 
diagrammatically  in  Fig.  i.  By  approximately  balancing 
the  load  between  the  two  circuits,  the  current  in  the 
neutral  conductor  is  very  small,  the  drop  of  voltage  so 
negligible,  and  the  distribution,  regarding  voltage  drop 
and  copper  economy,  so  takes  place  at  220  volts,  while 
the  lamps  operate  at  no  volts.  Even  where  a  separate 
transformer  feeds  a  single  house,  usually  a  three-wire 
distribution  is  preferable,  if  the  number  of  lamps  is  not 
very  small. 


FIG.  i. — Typical  three-wire  lighting  circuit. 

Two  hundred  and  twenty-volt  distribution,  with  440 
volts  between  the  outside  conductors,  has  been  tried  but 
found  uneconomical  in  this  country,  due  to  the  lower 
efficiency  of  the  2  20- volt  lamp. 

In  this  country,  no-volt  lamps  are  therefore  used 
almost  exclusively,  while  in  England,  for  instance,  22o-volt 
lamps  are  largely  used,  in  a  three- wire  distribution  system 
with  440  volts  between  the  outside  conductors.  The 
amount  of  copper  required  in  the  distribution  system, 
with  the  same  loss  of  power  in  the  distributing  conductors, 
is  inversely  proportional  to  the  square  of  the  voltage. 
That  is,  at  twice  the  voltage,  twice  the  voltage  drop  can 
be  allowed  for  the  same  distribution  efficiency;  and  as  at 
double  voltage  the  current  is  one-half,  for  the  same  load 


6  GENERAL  LECTURES 

twice  the  voltage  drop  at  half  the  current  gives  four  times 
the  resistance,  that  is,  one-quarter  the  conductor  material. 
By  the  change  from  the  2  20- volt  distribution  with  110- 
volt  lamps,  to  the  44o-volt  distribution  with  22o-volt 
lamps,  the  amount  of  copper  in  the  distributing  conductor, 
and  thereby  the  cost  of  investment  can  be  greatly  reduced, 
and  current  supplied  over  greater  distances,  so  that  from 
the  point  of  view  of  the  economical  supply  of  current  at 
the  customers'  terminals,  the  higher  voltage  is  preferable. 
However,  in  the  usual  sizes,  from  50  to  60  watts  power  con- 
sumption and  so  16  candlepower  with  the  old  carbon 
filament,  and  to  a  much  larger  extent  still  with  the  metal- 
filament  lamp,  as  the  tungsten  lamp,  the  22o-volt  lamp 
is  materially  less  efficient,  that  is,  requires  from  10  to  15 
per  cent,  more  power  than  the  no-volt  lamp,  when  pro- 
ducing the  same  amount  of  light  at  the  same  useful'  life. 
This  difference  is  inherent  in  the  incandescent  lamp,  and 
is  due  to  the  far  greater  length  and  smaller  section  of  the 
22o-volt  filament,  compared  with  the  no-volt  filament, 
and  therefore  no  possibility  of  overcoming  it  exists;  if  it 
should  be  possible  to  build  a  2  20- volt  lamp  as  efficient — 
at  the  same  useful  life — as  the  present  no-  volt  lamp,  this 
would  simply  mean,  that  by  the  same  improvement  the 
efficiency  of  the  no-volt  lamp  could  also  be  increased  from 
10  to  15  per  cent.,  and  the  difference  would  remain.  For 
small  units,  the  difference  in  efficiency  is  still  greater. 

Indeed,    in    England    and   those   countries   where   220- 
volt  distribution  is  extensively  used,  the  introduction  of 
the  tungsten  lamp — commonly  called   "mazda  lamp"— 
has  been,  and  still  is  very  seriously  retarded,  to  the  great 
disadvantage  of  the  user  of  the  light. 

The  loss  of  efficiency  of  10  to  15  per  cent.,  resulting 
from  the  use  of  the  22o-volt  lamp,  is  far  greater  than  the 


GENERAL  REVIEW  7 

saving  in  power  and  in  cost  of  investment  in  the  supply 
mains;  and  the  220-volt  system  with  no-volt  lamps  is 
therefore  more  efficient,  in  the  amount  of  light  produced 
in  the  customer's  lamps,  than  the  44o-volt  system  with 
2 20- volt  lamps.  In  this  country,  since  the  early  days,  the 
illuminating  companies  have  accepted  the  responsibility 
up  to  the  output  in  light  at  the  customer's  lamps,  by 
supplying  and  renewing  the  lamps  free  of  charge,  and 
the  system  using  no- volt  lamps  is  therefore  universally 
employed  while  the  2 20- volt  lamp  has  no  right  to  existence; 
while  abroad,  where  the  supply  company  considers  its 
responsibility  ended  at  the  customer's  meter,  and  the 
customer  is  left  to  supply  his  own  lamps,  the  supply 
company  saves  by  the  use  of  440-volt  systems — at  the 
expense  of  a  waste  of  power  in  the  customer's  2 20- volt 
lamps,  far  more  than  the  saving  effected  by  the  supply 
company. 

The  general  use  of  no-volt  distribution  in  this  country, 
and  the  extended  use  of  220  volts  abroad,  thus  is  the  result 
of  a  difference  in  the  policy  of  the  lighting  companies, 
more  particular  the  large  Edison  companies,  which  under 
the  inspiration  of  Edison  took  the  responsibility  of  giving 
the  customer  the  maximum  light  producible  at  a  given 
cost,  by  not  only  supplying  the  power,  but  also  the  lamps. 
The  policy  of  free  lamp  renewals  thus  has  been  the  most 
important  cause  which  has  brought  about  and  maintained 
the  superiority  of  the  lighting  service  of  the  American 
supply  companies.  With  the  introduction  of  the  metal- 
filament  lamp,  with  its  higher  cost  and  lesser  power  con- 
sumption, for  some  years  this  policy  of  free  lamp  renewals, 
on  which  the  American  superiority  is  based,  was  seriously 
threatened;  but  it  is  fortunate  that  by  cooperation  of  lamp 
manufacturer  and  supply  company  the  prices  of  tungsten 


8  GENERAL  LECTURES 

lamps  have  been  sufficiently  lowered,  and  their  life  in- 
creased, so  as  to  make  it  possible  to  return  to  free  lamp 
renewals  with  the  mazda  lamp,  and  thereby  safeguard  the 
quality  and  efficiency  of  the  lamps  used  by  the  customers, 
and  with  it  the  superior  quality  of  the  lighting  service. 

When  speaking  of  a  distribution  voltage  of  no,  some 
voltage  anywhere  in  the  range  from  105  to  130  volts  is 
meant,  and  the  various  distribution  systems  of  our  country 
have  chosen  various  voltages  within  this  wide  range,  to 
secure  best  economy  of  the  incandescent  lamp.  In  the 
manufacture  of  the  carbon-filament  lamp  of  old,  only 
two  of  the  three  quantities :  efficiency,  wattage  and  voltage, 
could  be  made  exact  according  to  specifications.  Thus  in 
treating  the  lamp,  its  voltage  could  be  made  right  at  the 
desired  wattage — but  then  the  efficiency  might  be  a  little 
off — or  the  desired  efficiency  could  be  produced  at  the 
desired  wattage — but  the  voltage  might  be  a  little  different 
from  specifications.  Thus  either  a  relatively  wide  margin 
would  have  to  be  allowed  in  complying  with  the  specifica- 
tions, that  is,  a  fairly  irregular  product  accepted,  or  a 
considerable  percentage  of  the  lamps  would  not  meet 
specifications,  resulting  in  increased  cost.  This  difficulty 
was  met  by  producing  the  lamps  exact  regarding  efficiency 
and  wattage,  and  then  assorting  them  by  voltage,  and 
by  agreement  between  lamp  manufacturer  and  lighting 
companies,  various  operating  voltages  were  chosen  by  the 
latter,  so  as  to  utilize  the  entire  lamp  product.  Hereby  a 
very  close  rating  of  the  lamps,  and  correspondingly  high 
uniformity  and  economy  of  the  lighting  systems  were 
secured,  but  this  feature  led  to  the  large  number  of  dis- 
tribution voltages. 

However,  the  carbon-filament  lamp  has  now  practically 
disappeared,  and  in  the  mazda  lamp,  the  tungsten  wire, 


GENERAL  REVIEW  9 

drawn  to  exact  diameter  and  cut  off  to  exact  length,  fixed 
definitely  the  efficiency,  wattage  and  voltage,  so  that 
the  need  of  the  many  distribution  voltages  has  disappeared, 
and  they  are  rather  a  serious  economic  disadvantage,  by 
requiring  numerous  lamp  voltage  standards.  Therefore, 
a  strong  movement  exists  to  reduce  the  distribution  volt- 
ages again  to  the  least  possible  number,  perhaps  three  or 
four,  and  correspondingly  reduce  the  number  of  lamp 
standards,  and  as  this  is  in  the  direction  of  increased 
economy,  it  will  undoubtedly  be  brought  about,  though  it 
will  probably  not  be  possible  to  go  to  one  single  standard 
lamp  voltage  only.  However,  sometimes  more  than  one 
standard  voltage  may  be  desirable,  as  in  an  extended 
distribution  system  a  higher  lamp  voltage  may  be  chosen 
as  standard  near  the  substation,  than  at  a  distance,  and 
the  problem  of  voltage  regulation  thereby  somewhat 
simplified. 

In  considering  distribution  systems,  it  therefore  is 
unnecessary  to  consider  any  other  lamp  voltage  than  no 
volts  (that  is,  the  range  of  voltage  represented  thereby). 

In  direct-current  distribution  systems,  as  used  in  most 
large  cities,  the  220-volt  network  is  fed  from  a  direct- 
current  generating  station,  or — as  now  more  frequently  is 
the  case — from  a  converter  substation,  which  receives  its 
power  as  three-phase  alternating,  from  the  main  generating 
station,  or  long-distance  transmission  line.  In  alternating- 
current  distribution,  the  220-volt  distribution  circuits  are 
fed  by  step-down  transformers  from  the  2  200- volt  primary 
distribution  system.  In  the  latter  case,  where  consider- 
able motor  load  has  to  be  considered,  some  arrangement 
of  polyphase  supply  is  desirable,  as  the  single-phase  motor  is 
inferior  to  the  polyphase  motor,  and  so  the  latter  is  pref- 
erable for  large  and  moderate  sizes. 


10  GENERAL  LECTURES 

COMPARISON  OF  ALTERNATING  CURRENT  AND  DIRECT 
CURRENT 

At  the  low  distribution  voltage  of  220,  current  can 
economically  be  supplied  from  a  moderate  distance  only, 
rarely  exceeding  from  i  to  2  miles.  In  a  direct-current 
system,  the  current  must  be  supplied  from  a  generating 
station  or  a  converter  substation,  that  is,  a  station  con- 
taining revolving  machinery.  As  such  a  station  requires 
continuous  attention,  its  operation  would  hardly  be 
economical  if  not  of  a  capacity  of  at  least  some  hundred 
kilowatts.  The  direct-current  distribution  system  there- 
fore can  be  used  economically  only  if  a  sufficient  demand 
exists,  within  a  radius  of  i  to  2  miles,  to  load  a  good- 
sized  generator  or  converter  substation.  The  use  of 
direct  current  is  therefore  restricted  to  those  places  where 
a  fairly  concentrated  load  exists,  as  in  large  cities;  while 
in  the  suburbs,  and  in  small  cities  and  villages,  where 
the  load  is  too  scattered  to  reach,  from  one  low-tension 
supply  point,  sufficient  customers  to  load  a  substation,  the 
alternating  current  must  be  used,  as  it  requires  merely  a 
step-down  transformer  which  needs  no  attention. 

However,  in  the  last  years,  some  automatic  converter 
substations  have  been  developed, which  operate  very  satis- 
factorily without  any  attendance  beyond  an  occasional 
inspection. 

In  the  interior  of  large  cities,  the  alternating-current 
system  is  at  a  disadvantage,  because  in  addition  to  the 
voltage  consumed  by  resistance,  an  additional  drop  of 
voltage  occurs  by  self-induction,  or  by  reactance;  and 
with  the  large  conductors  required  for  the  distribution  of 
a  large  low-tension  current,  the  drop  of  voltage  by  self- 
induction  is  far  greater  than  that  by  resistance,  and  the 
regulation  of  the  system  therefore  is  seriously  impaired, 


GENERAL  REVIEW  11 

or  at  least  the  voltage  regulation  becomes  far  more  difficult 
than  with  direct  current.  A  second  disadvantage  of 
the  alternating  current  for  distribution  in  large  cities  is, 
that  a  considerable  part  of  the  motor  load  is  elevator 
motors,  and  the  alternating-current  elevator  motor  is 
still  somewhat  inferior  to  the  direct-current  motor.  Ele- 
vator service  essentially  consists  in  starting  at  heavy  torque, 
and  rapid  acceleration,  and  in  both  of  these  features  the 
direct-current  motor  with  compound  field  winding  is 
superior,  and  easier  to  control. 

Where  therefore  direct  current  can  be  used  in  low-tension 
distribution;  it  is  preferable  to  use  it,  and  to  relegate 
alternating-current  low-tension  distribution  to  those  cases 
where  direct  current  cannot  be  used,  that  is,  where  the 
load  is  not  sufficiently  concentrated  to  economically  op- 
erate converter  substations. 

The  loss  of  power  in  the  low-tension  direct-current  system 
is  merely  the  izr  loss  in  the  conductors,  which  is  zero  at  no 
load,  and  increases  with  the  load ;  the  only  constant  loss  in 
a  direct-current  distribution  system  is  the  loss  of  power  in 
the  shunt  coils  of  the  integrating  wattmeters  on  the 
customer's  premises.  In  the  direct -current  system,  there- 
fore, the  efficiency  of  distribution  is  highest  at  light  load, 
and  decreases  with  increasing  load. 

In  an  alternating-current  distribution  system,  with  a 
2  2 oo- volt  primary  distribution,  feeding  secondary  low- 
tension  circuits  by  step-down  transformers,  the  i2r  loss 
in  the  conductors  usually  is  far  smaller  than  in  the  direct- 
current  system,  but  a  considerable  constant,  or  "no- 
load,"  loss  exists;  the  core  loss  in  the  transformers,  and 
the  efficiency  of  an  alternating-current  distribution  is 
usually  lowest  at  light  load,  but  increases  with  increase 
of  load,  since  with  increasing  load  the  transformer  core 


12  GENERAL  LECTURES 

loss  becomes  a  lesser  and  lesser  percentage  of  the  total 
power.  The  i*r  loss  in  alternating-current  systems  must 
be  far  lower  than  in  direct-current  systems: 

1.  Because  it  is  not  the  only  loss,  and  the  existence  of 
the  "no-load"  or  transformer  core  loss  requires  to  reduce 
the  load  loss  or  i*r  loss,  if  an  equally  good  efficiency  is 
desired.     With  an  alternating-current  system,  each  low- 
tension    main    requires    only    a    step-down    transformer, 
which  needs  no  attention;  therefore,   many  more  trans- 
formers can  be  used  than  rotary  converter  substations  in  a 
direct-current  system,  and  the  i*r  loss  is  then  reduced  by 
the  greatly  reduced  distance  of  secondary  distribution. 

2.  In  the  alternating-current  system,  the  drop  of  voltage 
in  the  conductors  is  greater  than  the  ir  drop  by  the  self- 
inductive  drop;  the  ir  drop  is  therefore  only  a  part  of  the 
total  voltage  drop;  and  with  the  same  voltage  drop  and 
therefore  the  same  regulation  as  a  direct-current  system, 
the  i2r  loss  in  the  alternating-current  system  would  be 
smaller  than  in  the  direct-current  system. 

3.  Due  to  the  self -inductive  drop,  smaller  and,  therefore, 
more  numerous  low-tension  distribution  circuits  must  be 
used  with  alternating  current  than  with  direct  current, 
and   a   separate   and   independent   voltage   regulation   of 
each  low-tension  circuit — that  is — each  transformer,  there- 
fore,   usually    becomes    impracticable.     This    means  that 
the  total  voltage  drop,  resistance  and  inductance,  in  the 
alternating-current  low-tension  distribution  circuits  must 
be  kept  within  a  few  per  cent.,  that  is,  within  the  range 
permissible  by  the  incandescent  lamp.     As  a  result  thereof, 
the    voltage    regulation    of    an    alternating-current    low- 
tension  distribution  has  frequently  been  inferior  to  that 
of  the  direct-current  distribution — in  many  cases  to  such 


GENERAL  REVIEW  13 

an  extent  as  to  require  the  use  of  incandescent  lamps  of 
lower  efficiency. 

However,  by  the  extended  use  of  voltage  regulators  in 
the  primary  alternating-current  feeders,  good  voltage 
regulation  is  secured  in  the  better  class  of  alternating- 
current  distribution  systems. 


SECOND  LECTURE 
GENERAL  DISTRIBUTION 

DIRECT-CURRENT  DISTRIBUTION 

The  typical  direct-current  distribution  is  the  system 
of  feeders  and  mains,  as  devised  by  Edison,  and  since  used 
in  all  direct-current  distributions.  It  is  shown  dia- 
grammatically  in  Fig.  2.  The  conductors  are  usually 
underground,  as  direct-current  systems  are  used  only  in 
large  cities.  A  system  of  three- wire  conductors,  called  the 
" mains"  is  laid  in  the  streets  of  the  city,  shown  dia- 
grammatically  by  the  heavily  drawn  lines.  Commonly, 
conductors  of  1,000,000  circular  mil  section  (that  is,  a 
copper  section  which  as  solid  round  conductor  would  have 
a  diameter  of  i")  are  used  for  the  outside  conductors,  the 
"positive"  and  the  "negative"  conductor;  and  a  con- 
ductor of  half  this  size  for  the  middle  or  "neutral"  con- 
ductor. The  latter  is  usually  grounded,  as  protection 
against  fire  risk,  etc.  Conductors  of  more  than  1,000,000 
circular  mils  are  generally  not  used,  but  when  the  load 
exceeds  the  capacity  of  such  conductors,  a  second  main 
is  laid  in  the  same  street.  A  number  of  feeders,  shown 
by  dotted  lines  in  Fig.  2,  radiate  from  the  generating 
station  or  converter  substations,  and  tap  into  the  mains  at 
numerous  points;  potential  wires  run  back  from  the  mains 
to  the  station,  and  so  allow  of  measuring,  in  the  station, 
the  voltage  at  the  different  points  of  the  distribution 
system.  All  the  customers  are  connected  to  the  mains, 
but  none  to  the  feeders.  The  mains  and  feeders  are 

14 


GENERAL  DISTRIBUTION 


15 


arranged  so  that  no  appreciable  voltage  drop  takes  place 
in  the  mains,  but  all  drop  of  voltage  occurs  in  the  feeders; 
and  as  no  customers  connect  to  the  feeders,  the  only 


FIG.   2. — Edison  system  of  feeders  and  mains. 

limit  to  the  voltage  drop  in  the  feeders  is  efficiency  of 
distribution.  The  voltage  at  the  feeding  points  into  the 
mains  is  kept  constant  by  varying  the  voltage  supply 
to  the  feeders  with  the  changes  of  the  load  on  the  mains. 


16 


GENERAL  LECTURES 


This  is  done  by  having  a  number  of  outside  busbars  in 
the  station,  as  shown  diagrammatically  in  Fig.  3,  differing 
from  each  other  in  voltage,  and  connecting  feeders  over 
from  busbar  to  busbar,  with  the  change  of  load. 

For  instance,  in  a  2  X  120  voltage  distribution,  the 
station  may  have,  in  addition  to  the  neutral  busbar  zero, 
three  positive  busbars  i,  i',  i",  and  three  negative  busbars 
2,  2',  2",  differing  respectively  from  the  neutral  bus  by 
120,  125  and  130  volts,,  as  shown  in  Fig.  3.  At  light  load, 
when  the  drop  of  voltage  in  the  feeders  is  negligible,  the 


z 

2' 


I  J 


ii    T 


FIG.  3. — Three- wire  direct-current  station. 

feeders  connect  to  the  buses  i,  o,  2  of  120  volts.  When 
the  load  increases,  some  of  the  feeders  are  shifted  over,  by 
transfer  busbars,  to  the  i25-volt  busbars  i'  and  2';  with 
still  further  increase  of  load,  more  feeders  are  connected 
over  to  125  volts;  then  some  feeders  are  connected  to  the 
i3o-volt  busbars,  i"  and  2",  and  so,  by  varying  the  vol- 
tage supply  to  the  feeders,  the  voltage  at  the  mains  can 
be  maintained  constant  with  an  accuracy  depending  on  the 
number  of  busbars.  It  is  obvious  that  a  shift  of  a  feeder 
from  one  voltage  to  another  does  not  mean  a  correspond- 


GENERAL  DISTRIBUTION  17 

ing  voltage  change  on  the  main  supplied  by  it,  but  rather 
a  shift  of  load  between  the  feeders,  and  so  a  readjustment 
of  the  total  voltage  in  the  territory  near  the  supply  point 
of  the  feeder.  For  instance,  if  by  the  potential  wires  a 
drop  of  voltage  below  120  volts  is  registered  in  the  main 
at  the  connection  point  of  feeder  A  in  Fig.  2,  and  this 
feeder  then  shifted  from  the  supply  voltage  125  to  130, 
the  current  in  the  main  near  A,  which  before  flowed  to- 
ward A  as  minimum  voltage  point,  reverses  in  direction, 
flows  away  from  A,  the  load  on  feeder  A  increases,  and, 
therefore,  the  drop  of  voltage  in  A  increases,  while  the 
load  on  the  adjacent  feeders  decreases,  and  thereby  their 
drop  of  Voltage  decreases,  with  the  result  of  bringing  up 
the  voltage  in  the  mains  at  the  feeder  A  and  all  adjacent 
feeders.  This  interlinkage  of  feeders,  therefore,  allows  a 
regulation  of  voltage  in  the  mains,  far  closer  than  the 
number  of  voltages  available  in  the  station. 

Originally,  such  direct-current  Edison  distribution  sys- 
tems were  fed  from  a  number  of  direct-current  generating 
stations,  having  machine  units — generally  direct-connected 
to  slow-speed  steam  engines — each  consisting  of  two  125- 
volt  generators,  connected  respectively  between  the  neutral 
and  the  two  outside  conductors  of  the  system.  Such 
direct-current  generating  systems  have  entirely  disap- 
peared, and  been  replaced  by  substations  fed  with  power 
from  one  or  a  number  of  high-power  high-voltage  three- 
phase  alternating  power-generating  stations  (except  in 
local  isolated  plants,  such  as  sometimes  used  in  office  build- 
ings, hotels,  etc.). 

In  the  substation,  three-wire  synchronous  converters 
are  most  frequently  used,  that  is,  25o-volt  converters 
in  which  the  neutral  is  brought  out  by  collector  rings 
and  derived  by  auto-transformer.  Sometimes  the  neutral 


18  GENERAL  LECTURES 

is  derived  by  a  separate  balancer  set,  a  pair  of  i25-volt 
machines  connected  in  series  between  the  three  lines,  or 
from  the  storage  battery;  induction  or  synchronous  motor- 
driven  direct-current  generators  are  also  occasionally 
used,  especially  where  the  alternating  supply  is  of  60 
cycles,  and  sometimes  two  separate  i25-volt  generators  or 
converters  in  series,  though  the  latter  arrangement  has 
practically  gone  out  of  use,  by  its  inefficiency. 

The  different  busbars  in  the  station  are  supplied  with 
their  voltage  by  having  different  generators  or  converters 
in  the  station  operate  at  different  voltages,  and  with 
increasing  load  on  the  station,  and  consequent  increasing 
demand  of  higher  voltage  by  the  feeders,  shift  machines  from 
lower  to  higher- volt  age  busbars,  inversely  with  decreasing 
load;  or  the  different  busbars  are  operated  through  boosters, 
or  by  connection  with  the  storage-battery  reserve,  etc. 

In  addition  to  feeders  and  mains,  tie  feeders  usually  con- 
nect the  generating  station  or  substation  with  adjacent 
stations,  so  that  during  periods  of  light  load,  or  in  case  of 
breakdown,  a  station  may  be  shut  down  altogether  and 
supplied  from  adjacent  stations  by  tie  feeders.  Such  tie 
feeders  also  permit  most  stations  to  operate  without 
storage-battery  reserve,  that  is,  to  concentrate  the  storage 
batteries  in  a  few  stations,  from  which  in  case  of  a  break- 
down of  the  system,  the  other  stations  are  supplied  over 
the  tie  feeders. 

All  more  important  direct-current  distribution  systems 
contain  a  storage-battery  reserve,  capable  of  maintaining 
service  in  case  of  complete  accident  and  shutdown,  until 
the  machinery  can  be  started  up  again. 

ALTERNATING-CURRENT  DISTRIBUTION 

The  system  of  feeders  and  mains  allows  the  most  perfect 
voltage  regulation  in  the  distributing  mains.  It  is,  how- 


GENERAL  DISTRIBUTION  19 

ever,  applicable  only  to  direct-current  distribution  in  a 
territory  of  very  concentrated  load,  as  in  the  interior  of  a 
large  city,  since  the  independent  voltage  regulation  of  each 
one  of  numerous  feeders  is  economically  permissible  only 
where  each  feeder  represents  a  large  amount  of  power ;  with 
alternating-current  systems,  the  inductive  drop  forbids  the 
concentration  of  such  large  currents  in  a  single  conductor. 
That  is,  conductors  of  1,000,000  circular  mils  cannot  be 
used  economically  in  an  alternating-current  system. 

The  resistance  of  a  conductor  is  inversely  proportional  to 
the  size  or  section  of  the  conductor,  hence  decreases  rapidly 
with  increasing  current:  a  conductor  of  1,000,000  circular 
mils  is  one- tenth  the  resistance  of  a  conductor  of  100,000 
circular  mils,  and  so  can  carry  ten  times  the  direct  current 
with  the  same  voltage  drop.  The  reactance  of  a  con- 
ductor,, however,  and  so  the  voltage  consumed  by  self- 
induction,  decreases  only  very  little  with  the  increasing 
size  of  a  conductor,  as  seen  from  the  table  of  resistances 
and  reactances  of  conductors  given  in  Appendix  II.  A 
wire  No.  ooo  B.  &  S.  gage  is  eight  times  the  section  of  a 
wire  No.  7,  and,  therefore,  one-eighth  the  resistance;  but 
the  wire  No.  ooo  has  a  reactance  of  0.109  ohrn  per  1000 
feet,  the  wire  No.  7  has  a  reactance  of  0.133  ohm,  or  only 
1.22  times  as  large.  Hence,  while  in  the  wire  No.  7,  the 
reactance,  at  60  cycles,  is  only  0.266  times  the  resistance 
and,  therefore,  not  of  serious  importance,  in  a  wire  No.  ooo 
the  reactance  is  1.76  times  the  resistance,  and  the  latter 
conductor  is  likely  to  give  a  voltage  drop  far  in  excess  of 
the  ohmic  resistance  drop.  The  ratio  of  reactance  to 
resistance,  therefore,  rapidly  increases  with  increasing 
size  of  conductor,  and  for  alternating  currents,  large 
conductors  cannot,  therefore,  be  used  economically  where 
close  voltage  regulation  is  required. 


20  GENERAL  LECTURES 

With  alternating  currents  it,  therefore,  is  preferable  to 
use  several  smaller  conductors  in  multiple :  two  conductors 
of  No.  i  in  multiple  have  the  same  resistance  as  one  con- 
ductor of  No.  ooo ;  but  the  reactance  of  one  conductor 
No.  ooo  is  0.109  ohm,  and  so  1.88  times  as  great  as  the 
reactance  of  two  conductors  of  No.  i  in  multiple,  which 
latter  is  half  that  of  one  conductor  No.  i,  or  0.058  ohm, 
provided  that  the  two  conductors  are  used  as  separate 
circuits. 

In  alternating-current  low-tension  distribution,  the  size 
of  the  conductor  and  so  the  current  per  conductor,  is 
limited  by  the  self -inductive  drop,  and  alternating-current 
low- tension  networks  are,  therefore,  of  necessity  of  smaller 
size  than  those  of  direct-current  distribution. 

As  regards  economy  of  distribution,  this  is  not  a  serious 
objection,  as  the  alternating-current  transformer  and 
primary  distribution  permits  the  use  of  numerous  second- 
ary circuits. 

In  alternating-current  systems,  a  primary  distribu- 
tion system  of  2200  volts  is  used,  feeding  step-down 
transformers. 

The  different  arrangements  are — 

(a)  A  separate  transformer  for  each  customer.  This  is 
necessary  in  those  cases  where  the  customers  are  so  far 
apart  from  each  other  that  they  cannot  be  reached  by  the 
same  low  tension  or  secondary  circuit;  every  alternating- 
current  system,  therefore,  has  at  least  a  number  of  in- 
stances where  individual  transformers  are  used. 

This  is  the  most  uneconomical  arrangement.  It  requires 
the  use  of  small  transformers,  which  are  necessarily  less 
efficient  and  more  expensive  per  kilowatt,  than  large  trans- 
formers. The  transformer  must  be  built  to  carry,  within 
its  overload  capacity,  all  the  lamps  installed  by  the 


GENERAL  DISTRIBUTION  21 

customer,  since  all  the  lamps  may  be  used  occasionally. 
Usually,  however,  only  a  small  part  of  the  lamps  are  in 
use,  and  those  only  for  a  small  part  of  the  day ;  so  that  the 
average  load  on  the  transformer  is  a  very  small  part  of  its 
capacity.  As  the  core  loss  in  the  transformer  continues 
whether  the  transformer  is  loaded  or  not,  but  is  not  paid 
for  by  the  customer,  the  economy  of  the  arrangement  is 
very  low;  and  so  it  can  be  understood  that  in  the  early 
days,  where  this  arrangement  was  generally  used,  the 
financial  results  of  most  alternating-current  distributions 
were  very  discouraging. 

Assuming  as  an  instance  a  connected  load  of  20:  60  watt 
lamps;  allowing  then  in  cases  of  all  lamps  being  used,  an 
overload  of  100  per  cent.,  which  is  rather  beyond  safe  limits, 
and  permissible  only  on  the  assumption  that  this  load  will 
occur  very  rarely,  and  for  a  short  time — the  transformer 
would  have  6oo-watt  rating.  Assuming  a  core  loss  of  4  per 
cent.,  this  gives  a  continuous  power  consumption  of  24  watts. 
Usually  probably  only  one  or  two  lamps  will  be  burning, 
and  these  only  a  few  hours  per  day,  so  that  the  use  of  two 
lamps,  at  an  average — summer  and  winter — of  3  hours  per 
day,  would  probably  be  a  fair  example  of  many  such 
cases.  Two  lamps  or  120  watts,  for  3  hours  per  day,  give 
an  average  power  of  15  watts,  which  is  paid  for  by  the 
customer,  while  the  continuous  loss  in  the  transformer  is 
24  watts;  so  that  the  all-year  efficiency,  or  the  ratio  of  the 
power  paid  for  by  the  customer,  to  the  power  consumed 

by  the  transformer,  is  only  - — ,        or  38  per  cent. 

By  connecting  several  adjacent  customers  to  the  same 
transformer,  the  conditions  immediately  become  far  more 
favorable.  It  is  extremely  improbable  that  all  the  cus- 
tomers will  burn  all  their  lamps  at  the  same  time,  the 


22  GENERAL  LECTURES 

more  so,  the  greater  the  number  of  customers  is,  which 
are  supplied  from  the  same  transformer.  It,  therefore, 
becomes  unnecessary  to  allow  a  transformer  capacity 
capable  of  operating  all  the  connected  load.  The  larger 
transformer  also  has  a  higher  efficiency.  Assuming,  there- 
fore, as  an  instance,  four  customers  of  20  lamps  connected 
load  each.  The  average  load  would  be  about  8  lamps. 
Assuming  even  one  customer  burning  all  20  lamps,  it  is 
not  probable  that  the  other  customers  together  would  at 
this  time  burn  more  than  10  to  15  lamps,  and  a  transformer 
carrying  30  to  35  lamps  at  overload  would  probably  'be 
sufficient.  A  i5oo-watt  transformer  would  therefore  be 
larger  than  necessary.  At  3  per  cent,  core  loss,  this  gives 
a  constant  loss  of  45  watts,  while  an  average  load  of  8 
lamps  for  3  hours  per  day  gives  a  useful  output  of  60  watts, 
or  an  all-year  efficiency  of  nearly  60  per  cent.,  while  a 
i ooo- watt  transformer  would  give  an  all-year  efficiency 
of  67  per  cent. 

This  also  illustrates  that  in  smaller  transformers  a  low 
core  loss  is  of  utmost  importance,  while  the  i2r  loss  is  of  very 
much  less  importance,  since  it  is  appreciable  only  at  heavy 
load,  and,  therefore,  affects  the  all-year  efficiency  very 
little. 

When  it  becomes  possible  to  connect  a  large  number  of 
customers  to  a  secondary  main  fed  from  one  large  trans- 
former the  connected  load  ceases  to  be  of  moment  in  the 
transformer  capacity;  the  transformer  capacity  is  deter- 
mined by  the  average  load,  with  a  safe  margin  for  over- 
loads; in  this  case,  good  all-year  efficiencies  can  be  reached, 
as  the  average  load  rarely  exceeds  one-third  of  the  con- 
nected load. 

Economical  alternating-current  distribution,  therefore, 
requires  the  use  of  secondary  distribution  mains  of  as 


GENERAL  DISTRIBUTION 


23 


large  an  extent  as  possible,  fed  by  large  transformers.  The 
distance,  however,  to  which  a  transformer  can  supply 
secondary  current,  is  rather  limited  by  the  inductive  drop 
of  voltage;  therefore,  for  supplying  secondary  mains, 
transformers  of  larger  size  than  60  kilowatts  are  rarely 
used,  but  rather  several  transformers  are  employed,  to 
feed  in  the  same  main  at  different  points. 


FIG.  4. — Alternating   current   distribution  with   secondary  mains  and  primary 

feeders. 

Extending  the  secondary  mains  still  further  by  the  use 
of  several  transformers  feeding  into  the  same  mains,  or,  as 
it  may  be  considered,  interconnecting  the  secondary  mains 
of  the  different  transformers,  we  arrive  at  a  system  some- 
what similar  to  the  direct-current  system:  a  low-tension 
distribution  system  of  220  volts  three- wire  mains,  with  a 
system  of  feeders  tapping  into  it  at  a  number  of  points,  as 
shown  in  Fig.  4.  These  feeders  are  primary  feeders  of 


24  GENERAL  LECTURES 

2200  volts,  connecting  to  the  mains  through  step-down 
transformers.  In  such  a  system,  by  varying  the  voltage 
impressed  upon  the  primary  feeders,  a  voltage  regulation 
of  the  system  similar  to  that  of  direct-current  distribution 
becomes  feasible.  Such  an  arrangement  has  the  ad- 
vantages over  the  direct-current  system,  that  the  drop 
in  the  feeders  is  very  much  lower,  due  to  their  higher 
voltage;  and  the  feeder  voltage  can  be  regulated  by  alter- 
nating-current feeder  regulators  or  auto- transformers,  that 
is,  stationary  structures  similar  to  the  transformer.  It 
has,  however,  the  disadvantage  that,  due  to  the  self- 
induction  of  the  mains,  each  feeding  point  can  supply 
current  over  a  far  shorter  distance  than  with  direct  current, 
and  the  interchange  of  current  between  feeders,  by  which 
the  load  can  be  shifted  and  apportioned  between  the  feeders, 
is  far  less. 

As  a  result,  it  is  difficult  to  reach  as  good  voltage  regu- 
lation with  the  same  attention  to  the  system;  and  since 
this  arrangement  has  the  disadvantage  that  any  break- 
down in  the  secondary  system  or  in  a  transformer  may 
involve  the  entire  system,  this  system  of  interconnected 
secondary  mains  is  rarely  used  for  alternating-current 
distribution,  but  the  secondary  mains  are  usually  kept 
separate.  That  is,  as  shown  diagrammatically  in  Fig.  5,  a 
number  of  separate  secondary  mains  are  fed  by  large  trans- 
formers from  primary  feeders,  and  usually  each  primary 
feeder  connects  to  a  number  of  transformers.  Where  the 
distances  are  considerable,  and  the  voltage  drop  in  the 
primary  feeders  appreciable,  voltage  regulation  of  the 
feeders  becomes  necessary;  and  in  this  case,  to  get  good 
voltage  regulation  in  the  system,  attention  must  be  given 
to  the  arrangements  of  the  feeders  and  mains.  That  is, 
all  the  transformers  on  the  same  feeder  should  be  at  about 


GENERAL  DISTRIBUTION 


25 


the  same  distance  from  the  station,  so  that  the  voltage 
drop  between  the  transformers  on  the  same  feeder  is 
negligible;  and  the  nature  of  the  load  on  the  secondary 


FIG.  5. — Typical  alternating-current  distribution. 

mains  fed  by  the  same  feeder  should  be  about  as  nearly  the 
same  as  feasible,  so  that  all  the  mains  on  the  same  feeder  are 
about  equally  loaded.  It  may,  therefore,  be  undesirable  for 
voltage  regulation,  to  connect,  for  instance,  a  main  feeding  a 


26  GENERAL  LECTURES 

residential  section  to  the  same  feeder  as  a  main  feeding  a 
business  district  or  an  office  building. 

In  a  well-designed  alternating-current  distribution  sys- 
tem, that  is,  a  system  using  secondary  distribution  mains 
as  far  as  feasible,  the  all-year  efficiency  is  about  the  same 
as  with  the  direct-current  system.  In  such  an  alternating- 
current  system,  the  efficiency  at  heavy  load  is  higher,  and 
at  light  load  lower,  than  in  the  direct-current  system;  in 
this  respect  the  alternating-current  system  has  the  ad- 
vantage over  the  direct-current  system,  since  at  the  time 
of  heavy  load  the  power  is  more  valuable  than  at  light 
load. 


THIRD  LECTURE 
LIGHT  AND  POWER  DISTRIBUTION 

In  a  direct-current  distribution  system,  the  motor  load 
is  connected  to  the  outside  mains  at  220  volts,  and  only 
very  small  motors,  as  fan  motors,  between  outside  mains 
and  neutral ;  since  the  latter  connection,  with  a  large  motor, 
would  locally  unbalance  a  system.  The  effect  of  a  motor 
on  the  system  depends  upon  its  size  and  starting  current, 
and  with  the  large  mains  and  feeders,  which  are  generally 
used,  even  the  starting  of  large  elevator  motors  has  no 
appreciable  effect,  and  the  supply  of  power  to  electric 
elevators  represents  a  very  important  use  of  direct-current 
distribution. 

In  alternating-current  distribution  systems,  the  effect  on 
the  voltage  regulation,  when  starting  a  motor,  is  more 
severe;  since  alternating-current  motors  in  starting  usually 
take  a  larger  current  than  direct-current  motors  starting 
with  the  same  torque  on  the  same  voltage;  and  the  current 
of  the  alternating-current  motor  is  lagging,  the  voltage 
drop  caused  by  it  in  the  reactance  is,  therefore,  greater  than 
would  be  caused  by  the  same  current  taken  by  a  non- 
inductive  load,  as  lamps.  Furthermore,  alternating-cur- 
rent supply  mains  usually  are  of  far  smaller  capacity,  and, 
therefore,  more  affected  in  voltage.  Large  motors  are, 
therefore,  rarely  connected  to  the  lighting  mains  of  an 
alternating-current  system,  but  separate  transformers  and 
frequently  separate  feeders  are  used  for  the  motors,  and 
very  large  motors  are  commonly  built  for  the  primary 

distribution  voltage  of  2200,  and  connected  to  these  mains. 

27 


28  GENERAL  LECTURES 

For  use  in  an  alternating-current  distribution  system,  the 
synchronous  motor  hardly  comes  into  consideration,  since 
the  synchronous  type  is  suitable  mainly  for  large  powers, 
where  it  is  operated  on  a  separate  circuit. 

The  alternating-current  motor  mostly  used  in  small  and 
moderate  sizes — such  as  come  into  consideration  for  power 
distribution  from  a  general  supply  system — is  the  induction 
motor  and,  where  high  torque  starting  and  acceleration  or 
adjustable  speed  are  desirable,  the  repulsion-induction 
motor,  which  is  a  single-phase  alternating  commutator 
motor.  The  single-phase  induction  motor,  however,  is  so 
inferior  to  the  polyphase  induction  motor,  that  single-phase 
motors  are  used  only  in  small  sizes ;  for  medium  and  larger 
sizes  the  three-phase  or  two-phase  motor  is  preferred  or  a 
commutator  motor  is  used.  The  former,  however,  in- 
troduces a  complication  in  the  distribution  system,  and 
the  three-wire  single-phase  system,  therefore,  is  less  suited 
for  motor  supply,  but  additional  conductors  have  to  be 
added  to  give  a  polyphase  power  supply  to  the  motor. 
As  the  result  thereof,  motors  are  not  used  in  alternating- 
current  systems  to  the  same  extent  as  in  direct-current 
systems.  In  the  alternating-current  system,  however, 
the  motor  load  is,  if  anything,  more  important  than  in  the 
direct-current  system,  to  increase  the  load  factor  of  the 
system;  since  the  efficiency  of  the  alternating-current 
system  decreases  with  decrease  of  load,  while  that  of  a 
direct-current  system  increases. 

Compared  with  the  direct-current  motor,  the  polyphase 
induction  motor  has  the  disadvantage  of  being  less  flexible : 
its  speed  cannot  be  varied  economically,  as  that  of  a  direct- 
current  motor  by  varying  the  field  excitation.  Speed 
variation  of  the  induction  motor  produced  by  a  rheostat 
in  the  armature  or  secondary  circuit  is  accomplished  by 


LIGHT  AND  POWER  DISTRIBUTION  29 

wasting  power:  the  power  input  of  an  induction  motor 
always  corresponds  to  full  speed;  if  the  speed  is  reduced 
by  running  on  the  rheostat,  the  difference  in  power  be- 
tween that  which  the  motor  actually  gives,  and  that  which 
it  would  give,  with  the  same  torque,  at  full  speed,  is  con- 
sumed in  the  rheostat. 

Where,  therefore,  different  motor  speeds  are  required, 
provisions  are  made  in  the  induction  motor  to  change  the 
number  of  poles;  thereby  a  number  of  different  definite 
speeds  are  available,  at  which  the  motor  operates  economic- 
ally as  "multispeed"  motor. 

The  starting  torque  of  the  polyphase  induction  motor 
with  starting  rheostat  in  the  armature  is  the  same  as  the 
running  torque  at  the  same  current  input,  just  as  in  the 
case  of  the  direct-current  shunt  motor  with  constant 
field  excitation.  In  the  squirrel-cage  induction  motor, 
however,  the  starting  torque  is  far  less  than  the  running 
torque  at  the  same  current  input;  or  inversely,  to  produce 
the  same  starting  torque,  a  greater  starting  current  is 
required.  In  starting  torque  or  current,  the  squirrel-cage 
induction  motor  has  the  disadvantage  against  the  direct- 
current  motor.  It  has,  however,  an  enormous  advantage 
over  it  in  its  greater  simplicity  and  reliability,  due  to  the 
absence  of  commutator  and  brushes,  and  the  use  of  a 
squirrel-cage  armature. 

The  advantage  of  simplicity  and  reliability  of  the  squir- 
rel-cage induction  motor  sufficiently  compensates  for  the 
disadvantage  of  the  large  starting  current,  to  make  the 
motor  most  commonly  used.  In  an  alternating-current 
distribution  system,  however,  great  care  has  to  be  taken 
to  avoid  the  use  of  such  larger  motors  at  places  where  their 
heavy  lagging  starting  currents  may  affect  the  voltage 


30  GENERAL  LECTURES 

regulation;  in  such  places,  separate  transformers  and  even 
separate  primary  feeders  are  desirable. 

The  single-phase  induction  motor  is  not  desirable  in 
larger  sizes  in  a  distribution  system,  since  its  starting 
current  is  still  larger;  in  small  sizes,  however,  it  is  ex- 
tensively used,  since  it  requires  no  special  conductors, 
but  can  be  operated  from  a  single-phase  lighting  main. 

The  alternating-current  commutator  motor  is  a  single- 
phase  motor  which  has  all  the  advantages  of  the  different 
types  of  direct-current  motors;  it  can  be  built  as  constant- 
speed  motor  of  the  shunt  type,  or  as  motor  with  the  char- 
acteristics of  the  direct-current  series  motor:  very  high 
starting  torque  with  moderate  starting  current.  It  has, 
however,  also  the  disadvantages  of  the  direct-current 
motor:  commutator  and  brushes;  and  so  requires  more 
attention  than  the  squirrel-cage  induction  motor. 

However,  this  disadvantage  usually  is  not  very  serious, 
and  the  advantages  of  such  alternating-current  commu- 
tator motors:  to  operate  on  single-phase  distribution 
circuits,  to  give  high  starting  torque  efficiency,  that  is, 
start  under  load  with  moderate  current,  to  allow  efficient 
speed  variation  by  what  in  its  principle  amounts  to  field 
control,  and  the  possibility  of  giving  very  high  power 
factors,  has  led  to  the  development  and  extensive  intro- 
duction of  such  single-phase  commutator  motors,  usually  of 
the  type  of  the  repulsion  motor  or  the  compensated  re- 
pulsion motor. 

Alternating-current  generators  now  are  almost  always 
used  as  three-phase  machines,  and  transmission  lines  are 
always  three-phase,  though  in  transforming  down,  the 
system  can  be  changed  to  two-phase.  The  power  supply 
in  an  alternating-current  system,  therefore,  is  practically 
always  polyphase;  and  since  a  motor  load,  which  is  very 


LIGHT  AND  POWER  DISTRIBUTION  31 

desirable  for  economical  operation,  also  requires  polyphase 
currents,  alternating-current  distribution  systems  always 
start  from  polyphase  power. 

The  problem  of  alternating-current  distribution,  there- 
fore, is  to  supply,  from  a  polyphase  generating  system, 
single-phase  current  to  the  incandescent  lamps,  and  poly- 
phase current  to  the  induction  motors. 

PRIMARY  DISTRIBUTION  SYSTEMS 

1.  Two   conductors   of   the   three-phase   generating   or 
transmission  system  are  used  to  supply  a  2200  single-phase 
system  for  lighting  by  step-down  transformers  and  three- 
wire  secondary  mains;  the  third  conductor  is  carried  to 
those  places  where  motors  are  used  and  three-phase  motors 
are    operated    by    separate    step-down    transformers.     In 
the  lighting  feeders,  the  voltage  is  then  controlled  by  feeder 
regulators,  or,  in  a  smaller  system,  the  generator  excita- 
tion is  varied  so  as  to  maintain  the  proper  voltage  on  the 
lighting   phase.     At   load,    the   three-phase   triangle   then 
more  or  less  unbalances,  but  induction  motors  are  very 
little  sensitive  to  unbalancing  of  the  voltage,  and  by  their 
regulation — by   taking   more   current   from   the   phase  of 
higher,    less   from   the   phase   of   lower   voltage — tend   to 
restore  the  balance.     For  smaller  motors,  frequently  two 
transformers  are  used,  arranged  in  "open  delta"  connection. 

2.  Two-phase  generators  are  used,  or  in  the  step-down 
transformers  of  a  three-phase  transmission  line,  the  vol- 
tage is  changed  from  three-phase  to  two-phase ;  the  lighting 
feeders  are  distributed  between  the  two  phases  and  con- 
trolled by  potential  regulators  so  that  the  distribution  for 
lighting  is  single-phase,   by  three-wire  secondary  mains. 
For  motors,  both  phases  are  brought  together,   and  the 
voltage  stepped  down  for  use  on  two-phase  motors.     This 


32  GENERAL  LECTURES 

requires  four,  or  at  least  three,  primary  wires  to  motor 
loads.     This  system  is  only  rarely  used  today. 

3.  From  three-phase  generators  or  transmission  lines, 
three  separate  single-phase  systems  are  operated  for  light- 
ing ;  that  is  the  lighting  feeders  with  their  voltage  regulators 
are  distributed  between  the  three  phases,   and  all  three 
primary  wires  are  brought  to  the  step-down  transformers 
for  motors.     This  arrangement,  by  distributing  the  light- 
ing feeders  between  the  three  phases,  would  require  more 
care  in  exactly  balancing  the  load  between  all  three  phases 
than  two,  but  a  much  greater  unbalancing  can  be  allowed 
without  affecting  the  voltage.     Separate  feeder  regulators 
then  are  used  in  the  three  phases. 

4.  Four- wire    three-phase    primary    distribution    with 
neutral  wire,  which  sometimes  is  grounded,  and  2200  volts 
between   outside   conductors   and   neutral.     The   lighting 
feeders  are  distributed  between  the  three  circuits  between 
outside  conductors  and  neutral,  and  motors  supplied  by 
three    of   such   transformers.     This    system   is   now   very 
frequently  used,  and  is  becoming  of  increasing  importance, 
since  it  allows  economical  distribution  to  distances  beyond 
those  which  can  be  reached  with  2200  volts:  with  2600 
volts  on  the  transformers — as  the  upper  limit  of  primary 
distribution   voltage — the   voltage   between    outside    con- 
ductors is  4500,  and  the  copper  economy  of  the  system, 
therefore,  is  that  of  a  4 500- volt  three-phase  system. 

5.  Polyphase  primary  and  polyphase  secondary  distri- 
bution, with  the  motors  connected  to  the  same  secondary 
mains   as  the  lights.     This   system  is  largely  used  only 
where  most  of  the  load  is  power  distribution,  as  in  factories, 
etc. 

6.  Six   thousand   six   hundred   volts   and    13,200   volts 
single-phase  and  three-phase  are  increasingly  being  used 


LIGHT  AND  POWER  DISTRIBUTION  33 

in  primary  distribution  in  less  densely  populated  territories 
and  with  power  derived  from  a  very  high-voltage  trans- 
mission line,  especially  in  the  West. 

In  supplying  villages  and  other  small  settlements  with 
electric  power  from  a  very  high-voltage  transmission  line, 
100,000  volts  for  instance,  the  difficulty  is  that  trans- 
formation from  such  high  voltages  becomes  economical 
only  in  larger  units,  hundreds  of  kilowatts,  and  the  power 
demand  of  a  village  or  smaller  settlement  is  rarely  as 
large  as  this,  thus  can  be  economically  met  only  by  choosing 
a  primary  distribution  voltage  sufficiently  high  to  supply  a 
number  of  places  from  the  same  step-down  transformer. 
Six  thousand  six  hundred  volts  and  in  less  densely  popu- 
lated districts  13,200  volts  have  been  found  well  suited  for 
this  purpose. 

SYSTEMS  OF  LOW-TENSION  DISTRIBUTION  FOR  LIGHTING  AND 

POWER 

i.  Two-wire,  Direct-current  or  Single-phase,  no  Volts 

(Fig.  6). — This  can  be  used  only  for  very  short  distances, 


i 

//Ory' 


FIG.  6. — Two-wire  system. 


34 


GENERAL  LECTURES 


since  its  copper  economy  is  very  low,  that  is,  the  amount 
of  conductor  material  is  very  high  for  a  given  ^power. 

In  comparing  the  copper  efficiency  of  different  systems 
this  usually  is  considered  as  unity,  that  is:  Cu  i 

2.  Three-wire,  Direct -current  or  Single-phase,  no  to 
220  Volts  (Fig.  7). — Neutral  one-half  size  of  the  two  outside 


F      1 

*—  220* 


FIG.  7. — Three-wire  system. 


conductors.  The  two  outside  conductors  require  one- 
quarter  the  copper  of  the  two  wires  of  a  no-volt  system; 
since  at  twice  the  voltage  and  one-half  the  current,  four 
times  the  resistance  or  one-quarter  the  copper  is  sufficient 
for  the  same  loss  (the  amount  of  conductor  material 
varying  with  the  square  of  the  voltage). 

Adding  then  one-quarter  for  the  neutral  of  half -size,  gives 
K  X  M  =  KG  or  altogether  K  +  KG  =  KG  of  the  con- 
ductor material  required  by  the  two-wire  no-volt  system. 


LIGHT  AND  POWER  DISTRIBUTION  35 

That  is,  the  copper  economy  is  Jf  6-  This  is  the  most  com- 
monly used  system,  since  it  is  very  economical,  and  requires 
only  three  conductors.  It  is,  however,  a  single-phase  sys- 
tem, and,  therefore,  not  suitable  for  operating  polyphase 
induction  motors.  Cu  £{6 

3.  Four -wire    Quarter-phase    (Two-phase)    (Fig.  8).— 
Two  separate  two-wire  single-phase  circuits,  therefore,  no 


ok 
o 

0 

G> 

f 

//0V 

0 

o 
o( 

<=> 
<=> 

t 

//0/K 
4 

PIG.  8 — Four-wire  two-phase  system. 

saving  in  copper  over  two- wire  systems.  That  is,  the  cop- 
per economy  is :  Cu  i 

This  system  is  only  little  used. 

4.  Three-wire  Quarter -phase  (Fig.  9). — Common  return 
of  both  phases,  therefore,  saves  one  wire  or  one-quarter  of 
the  copper ;  hence  has  the  copper  economy :  Cu  % 


FIG.  9. — Three-wire  two-phase  system. 

In  this  case,  however,  the  middle  or  common  return  wire 
carries  \/2,  or  1.41  times  as  much  current  as  the  other  two 
wires,  and  when  making  all  three  wires  of  the  same  size,  the 


36 


GENERAL  LECTURES 


copper  is  not  used  most  economically.  A  small  further 
saving  is,  therefore,  made  by  increasing  the  middle  wire  and 
decreasing  the  outside  wires  so  that  the  middle  wire  has 
1.41  times  the  section  of  each  outside  wire.  This  improves 
the  copper  economy  to:  Cu  0.73 

This  system  is  used  in  special  cases  only. 

5.  Three-wire  Three-phase  (Fig.  10). — A  three-phase 
system  is  best  considered  as  a  combination  of  three  single- 
phase  systems,  of  the  voltage  from  line  to  neutral,  and  with 
zero  return  (because  the  three  currents  neutralize  each 
other  in  the  neutral). 


FIG.   10. — Three- wire  three-phase  system. 

Compared  thereto  the  two-wire  single-phase  system  can 
be  considered  as  a  combination  of  two  single-phase  circuits 
from  wire  to  neutral  with  zero  return. 

In  a  no-volt  single-phase  system  the  voltage  from  line  to 

no 

neutral  equals  11%,  in  a  three-phase  system  equals  -—=- 

V3 


The  ratio  of  voltages  is 


110 


II0    nr  IIQX 


-          2x110 

and  the  square  of  the  ratio  of  voltages  equals  %;  and  as  the 
copper  economy  varies  with  the  square  of  the  voltage,  the 
copper  economy  for  the  three-wire  three-phase  system  is: 

Cu  % 
This  system  is  going  out  of  use. 


LIGHT  AND  POWER  DISTRIBUTION 


37 


6.  Five-wire  Quarter-phase  (Fig.  n). — Neglecting  the 
neutral  conductor,  the  five-wire  quarter-phase  system  can 
be  considered  as  four  single-phase  circuits  without  return, 
from  line  to  neutral,  of  voltage  no.  Compared  with  the 
two- wire  circuit,  which  consists  of  two  single-phase  circuits 


T 


**€E 


i  i 


FIG.   ii.  —  Five-wire  two-phase  system. 


without  return,  of  n%  volts,  No.  6,  therefore,  has  twice 
the  voltage  of  No.  i  ;  therefore,  one-quarter  the  copper. 

Making  the  neutral  half  the  size  of  the  main  conductor 
adds  one-half  of  the  copper  of  one  conductor,  or  %  of  Y±  = 
J£2»  so  giving  a  total  of  y±  +  J^2>  that  is,  a  copper  economy 
of:  Cu%2 

Due  to  the  large  number  of  conductors  required,  this 
system  is  rarely  used. 

7.  Four  -wire  Three-phase  (Fig.  12).  —  Lamps  connected 
between  line  and  neutral. 


0 

T        f 

tf/J/x 
//oiG/r 

O/v 

• 

o 
o 
>- 

JL    \  « 
/wi  \  //oa, 

o 
o 

f        t 

T         i    , 

FIG.   12. — Four- wire  three-phase  system. 

Neglecting   the   neutral,    the   system   consists   of   three 
single-phase  circuits  without  return,  of  no  volts,  and  com- 


38' 


GENERAL  LECTURES 


pared  with  the  two-  wire  circuit  of  1X%  between  wire  and 
neutral  without  return,  it,  therefore,  requires  one-quarter 
the  copper. 

Making  the  neutral  one-half  size  adds  Y§  of  the  copper, 
or  Y§  of  J4  =  /^4i  and  so  gives  a  total  copper  economy 

Of  l/24  +  lA  =   %*•  CU  ^4 

This  system  is  used  to  some  extent,  especially  where  most 
of  the  load  is  power;  its  use,  however,  is  becoming  less 
frequent,  and  in  its  place  a  three-  wire  single-phase  system 
with  separate  three-phase  motor  mains  is  usually  employed 
now  for  factory  and  mill  work. 

8.  Three-wire  Single-phase  Lighting  with  Three-phase 
Power  (Fig.  13).  —  Lighting.  —  Half-size  neutral  same  as 
No.  2,  therefore,  copper  economy:  Cu 


o 

0 

o 

o 

x=» 

f        T 

220 

'; 

^_~ 

^ 

4 

t 

2On 
\ 

o 
o 

Q 

o 

£p" 

FIG.   13. — Single-phase  lighting  and  three-phase  power 

Power. — Three- wire  three-phase  220  volts;  that  is,  the 
same  as  No.  5,  but  twice  the  voltage,  thus  one-quarter  the 
copper  of  No.  5,  or  y±  of  %  =  %6:  Cu  %$ 

This  system  is  used  very  extensively. 

The  systems  mostly  used  are: 

No.  2.  Three-wire  direct  current  or  alternating-current 
single-phase. 

No.  8.  Three- wire  lighting,  three-phase  power.  Less 
frequent. 

No.  6.     Five- wire  quarter-phase. 

No.  7.     Four- wire  three-phase. 


LIGHT  AND  POWER  DISTRIBUTION  39 

As  we  have  seen,  the  two- wire  system  is  rather  inefficient 
in  copper.  High  efficiency  requires  the  use  of  a  third 
conductor,  that  is,  the  three-wire  system,  for  direct  current 
or  single-phase  alternating  current. 

Three-wire  polyphase  systems,  however,  are  inefficient  in 
copper,  as  No.  4  and  No.  5 ;  and  to  reach  approximately  the 
same  copper  economy,  as  is  reached  by  a  three- wire  system 
with  direct  current  and  single-phase  alternating  current,  re- 
quires at  least  four  wires  with  a  polyphase  system. 

That  is,  for  equal  economy  in  conductor  material,  the 
polyphase  system  requires  at  least  one  more  conductor  than 
the  single-phase  or  the  direct-current  distribution  system. 

While  the  field  of  direct-current  distribution  is  found 
in  the  interior  of  large  cities,  alternating  current  is  used  in 
smaller  towns  and  villages  and  in  the  suburbs  of  large  cities. 
In  the  latter,  therefore,  alternating  current  does  the  pioneer 
work.  That  is,  the  district  is  developed  by  alternating 
current,  usually  with  overhead  conductors,  and  when  the 
load  has  become  sufficiently  large  to  warrant  the  establish- 
ment of  converter  substations,  direct-current  mains  and 
feeders  are  laid  under  ground,  the  alternating-current  dis- 
tribution is  abandoned,  and  the  few  alternating-current 
motors  are  replaced  by  direct-current  motors.  In  the  last 
years,  however,  considerable  motor  load  has  been  developed 
in  the  alternating-current  suburban  distribution  systems, 
fairly  satisfactorily  alternating-current  elevator  motors 
have  been  developed  and  introduced  and  the  motor  load 
has  become  so  large  as  to  make  it  economically  difficult 
to  replace  the  alternating-current  motors  by  direct-current 
motors  in  changing  the  system  to  direct  current;  and  it, 
therefore,  appears  that  the  distribution  systems  of  large 
cities  will  be  forced  to  maintain  alternating-current  dis- 


40  GENERAL  LECTURES 

tribution  even  in  districts  of  such  character  as  would  make 
direct  current  preferable. 

As  the  result  therefrom,  direct-current  distribution  sys- 
tems increase  much  less  rapidly  than  alternating-current 
systems,  and  the  alternating-current  distribution  thus  is 
gaining  ground,  and  new  direct-current  distribution  systems 
are  hardly  ever  established  in  cities,  etc.,  but  the  direct- 
current  generator  finds  its  field  in  isolated  stations,  such 
as  installed  in  office  buildings,  theatres,  apartment  houses, 
hotels,  etc. 


FOURTH  LECTURE 
LOAD  FACTOR  AND  COST  OF  POWER 

The  cost  of  the  power  supplied  at  the  customer's  meter, 
consists  of  three  parts. 

A.  A  fixed  cost,  that  is,  cost  which  is  independent  of  the 
amount  of  power  used,  or  the  same  whether  the  system  is 
fully  loaded  or  carries  practically  no  load.     Of  this  char- 
acter, for  instance,  is  the  interest  on  the  investment  in  the 
plant,  the  salaries  of  its  officers,  etc. 

B.  A  cost  which  is  proportional  to  the  amount  of  power 
used.     Such  a  proportional  cost,  for  instance,  is  that  of 
fuel  in  a  steam  plant. 

C.  A  cost  depending  on  the  reliability  of  service  required, 
as  the  cost  of  keeping  a  steam  reserve  in  a  water-power 
transmission,  or  a  storage-battery  reserve  in  a  direct-current 
distribution. 

Since  of  the  three  parts  of  the  cost,  only  one,  B,  is  pro- 
portional to  the  power  used,  hence  constant  per  kilowatt 
output — the  other  two  parts  being  independent  of  the 
output — hence  the  higher  per  kilowatt,  the  smaller  a  part 
of  the  capacity  of  the  plant  the  output  is;  it  follows  that 
the  cost  of  power  delivered  is  a  function  of  the  ratio  of 
the  actual  output  of  the  plant,  to  the  available  capacity. 

Interest  on  the  investment  of  developing  the  water  power 
or  building  the  steam  plant,  the  transmission  lines,  cables 
and  distribution  circuits,  and  depreciation  are  items  of  the 
character  A,  or  fixed  cost,  since  they  are  practically  in- 
dependent of  the  power  which  is  produced  and  utilized. 

41 


42  GENERAL  LECTURES 

Fuel  in  a  steam  plant,  oil,  etc.,  are  proportional  costs,  that 
is,  essentially  depending  on  the  amount  of  power  produced. 

Salaries  are  fixed  cost,  A;  labor,  attendance  and  inspec- 
tion are  partly  fixed  cost  A,  partly  proportional  cost  B — 
economy  of  operation  requires,  therefore,  a  shifting  of  as 
large  a  part  thereof  over,  into  class  B,  by  shutting  down 
smaller  substations  during  periods  of  light  load,  etc. 

Incandescent-lamp  renewals,  arc-lamp  trimming,  etc., 
are  essentially  proportional  costs,  B. 

The  reserve  capacity  of  a  plant,  the  steam  reserve  main- 
tained at  the  receiving  end  of  a  transmission  line,  the  differ- 
ence in  cost  between  a  duplicate-pole  line  and  a  single-pole 
line  with  two  circuits,  the  storage-battery  reserve  of  the 
distribution  system,  the  tie  feeders  between  stations,  etc., 
are  items  of  the  character  C ;  that  is,  part  of  the  cost  insur- 
ing the  reliability  and  continuity  of  power  supply. 

The  greater  the  fixed  cost  A  is,  compared  with  the  pro- 
portional cost  B,  the  more  rapidly  the  cost  of  power  per  kilo- 
watt output  increases  with  decreasing  load.  Even  in  steam 
plants  very  frequently  A  is  larger  than  B,  that  is,  fuel,  etc., 
not  being  the  largest  items  of  cost;  in  water-power  plants 
A  practically  always  is  far  larger  than  B.  As  result  thereof, 
while  water  power  may  appear  very  cheap  when  consider- 
ing only  the  proportional  cost  B — which  is  very  low  in 
most  water  powers — the  fixed  cost  A  usually  is  very  high, 
due  to  the  hydraulic  development  required.  The  differ- 
ence in  the  cost  of  water  power  from  that  of  steam  power, 
therefore,  is  far  less  than  appears  at  first.  As  water  power 
is  usually  transmitted  over  a  long-distance  line,  while 
steam  power  is  generated  near  the  place  of  consumption, 
water  power  usually  is  far  less  reliable  than  steam  power. 
To  insure  equal  reliability,  a  water-power  plant  brings  the 
item  C,  the  reliability  cost,  very  high  in  comparison  with 


LOAD  FACTOR  AND  COST  OF  POWER  43 

the  reliability  cost  of  a  steam-power  plant,  since  the  pos- 
sibility of  a  breakdown  of  a  transmission  line  requires  a 
steam  reserve,  and  where  absolute  continuity  of  service 
is  required,  it  requires  also  a  storage  battery,  etc. ;  so  that 
on  the  basis  of  equal  reliability  of  service,  sometimes  very 
little  difference  in  cost  exists  between  steam  power  and 
water  power,  unless  the  hydraulic  development  of  the  latter 
was  very  simple,  and  some  very  large  steam-turbine  plants 
are  more  economical  in  electric-power  production,  than 
most  water-power  plants. 

The  cost  of  electric  power  of  different  systems,  therefore, 
is  not  directly  comparable  without  taking  into- considera- 
tion the  reliability  of  service  and  the  character  of  the  load. 

As  a  very  large,  and  frequently  even  the  largest  part  of 
the  cost  of  power,  is  independent  of  the  power  utilized,  and, 
therefore,  rapidly  increases  with  decreasing  load  on  the 
system,  the  ratio  of  average  power  output  to  the  available 
power  capacity  of  the  plant  is  of  fundamental  importance 
in  the  cost  of  power  per  kilowatt  delivered.  This  ratio, 
of  the  average  power  consumption  to  the  available  power, 
or  station  capacity,  has  occasionally  been  called  "load 
factor."  This  definition  of  the  term  "load  factor"  is, 
however,  undesirable,  since  it  does  not  take  into  considera- 
tion the  surplus  capacity  of  the  station,  which  may  have 
been  provided  for  future  extension ;  the  reserve  for  insuring 
reliability  C,  etc. ;  and  other  such  features  which  have  no 
direct  relation  whatever  to  the  character  of  the  load. 

Therefore,  as  load  factor  is  generally  understood  the 
ratio  of  the  average  load  to  the  maximum  load;  any  excess 
of  the  station  capacity  beyond  the  maximum  load  is  power 
which  has  not  yet  been  sold,  but  which  is  still  available  for 
the  market,  or  which  is  held  in  reserve  for  emergencies,  is 
not  charged  against  the  load  factor. 


44 


GENERAL  LECTURES 


The  cost  of  electric  power  essentially  depends  on  the  load 
factor.  The  higher  the  load  factor,  the  less  is  the  cost  of  the 
power,  and  a  low  load  factor  means  -an  abnormally  high  cost 
per  kilowatt.  This  is  the  case  in  steam  power,  and  to  a 
still  greater  extent  in  water  power. 

For  the  economical  operation  of  a  system,  it  therefore  is 
of  greatest  importance  to  secure  as  high  a  load  factor  as 
possible,  and  consequently,  the  cost — and  depending 
thereon  the  price — of  electric  power  for  different  uses  must 


i 


10 


1.0 


FIG.  14. — Summer  lighting  load  curve  and  factory  motor  load  curve. 

be  different  if  the  load  factors  are  different,  and  the  higher 
the  cost,  the  lower  the  load  factor. 

Electrochemical  work  gives  the  highest  load  factor, 
frequently  some  90  per  cent.,  while  a  lighting  system 
shows  the  poorest  load  factor — in  an  alternating-current 
system  without  motor  load  occasionally  it  is  as  low  as  10 
to  20  per  cent. 

Defining  the  load  factor  as  the  ratio  of  the  average  to 
the  maximum  load,  it  is  necessary  to  state  over  how  long  a 


LOAD  FACTOR  AND  COST  OF  POWER 


45 


time   the    average   is   extended;    that   is,    whether   daily, 
monthly  or  yearly  load  factor. 

For  instance,  Fig.  14  shows  an  approximate  load  curve 
of  a  lighting  circuit  during  a  summer  day;  practically  no 
load  except  for  a  short  time  during  the  evening,  where  a 


'/A 


Lo, 


\\ 


FIG.   15. — Winter  lighting  load  curve  and  factory  motor  load  curve 

high  peak  is  reached.  The  ratio  of  the  average  load  to  the 
maximum  load  during  this  day,  or  the  daily  load  factor,  is 
22.8  per  cent. 

Fig.  15  shows  an  approximate  lighting  load  curve  for  a 
winter  day:  a  small  maximum  in  the  morning,  and  a  very 
high  evening  maximum,  of  far  greater  width  than  the 


46  GENERAL  LECTURES 

summer  day  curve,  giving  a  daily  load  factor  of  34.5  per 
cent. 

During  the  year,  the  daily  load  curve  varies  between  the 
extremes  represented  by  Figs.  14  and  15,  and  the  average 
annual  load  is  therefore  about  midway  between  the  average 
load  of  a  summer  day  and  that  of  a  winter  day.  The 
maximum  yearly  load,  however,  is  the  maximum  load  dur- 
ing the  winter  day ;  and  the  ratio  of  average  yearly  load  to 
maximum  yearly  load,  or  the  yearly  load  factor  of  the 
lighting  system,  therefore  is  far  lower  than  the  daily  load 
factor:  if  we  consider  the  average  yearly  load  as  the 
average  between  14  and  15,  the  yearly  load  factor  is  only 
23.6  per  cent. 

One  of  the  greatest  disadvantages  of  lighting  distribu- 
tion, therefore,  is  the  low  yearly  load  factor,  resulting  from 
the  summer  load  being  so  very  far  below  the  winter  load; 
economy  of  operation,  therefore,  makes  an  increase  of  the 
summer  lighting  load  very  desirable.  This  has  led  to  the 
development  of  spectacular  lighting  during  the  summer 
months,  as  represented  by  the  various  Luna  Parks,  Dream- 
lands, etc. 

The  load  curve  of  a  factory  motor  load  is  about  the 
shape  shown  in  Fig.  16:  fairly  constant  from  the  opening 
of  the  factories  in  the  morning  to  their  closing  in  the  even- 
ing, with  perhaps  a  drop  of  short  duration  during  the  noon 
hour,  and  a  low  extension  in  the  evening,  representing 
overtime  work.  It  gives  a  daily  load  factor  of  49.5  per 
cent. 

This  load  curve,  superimposed  upon  the  summer  lighting 
curves,  does  not  appreciably  increases  the  maximum,  but 
very  greatly  increases  the  average  load,  as  shown  by  the 
dotted  curve  in  Fig.  14;  and  so  improves  the  load  factor, 
to  65.4  per  cent. — thereby  greatly  reducing  the  cost  of  the 


LOAD  FACTOR  AND  COST  OF  POWER 


47 


power  to  the  station,  in  this  way  showing  the  great  im- 
portance of  securing  a  large  motor  load.  During  the  winter 
months,  however,  the  motor  load  overlaps  the  lighting 
maximum,  as  shown  by  the  dotted  curve  in  Fig.  15.  This 
increases  the  maximum,  and  thereby  increases  the  load 
factor  less,  only  to  41.7  per  cent.  This  is  not  so  serious  in 
the  direct-current  system  with  storage-battery  reserve,  as 
the  overlap  extends  only  for  a  short  time,  the  overload 
being  taken  care  of  by  storage  batteries  or  by  the  overload 


FIG.   16. — Factory  power  load  curve. 

capacity  of  generators  and  steam  boilers;  but  where  it  is 
feasible,  it  is  a  great  advantage  if  the  users  of  motors  can 
be.  induced  to  shut  them  down  in  winter  with  beginning 
darkness. 

It  follows  herefrom,  that  additional  load  on  the  station 
during  the  peak  of  the  load  curve  is  very  expensive,  since  it 
increases  the  fixed  cost  A  and  C,  while  additional  load 
during  the  periods  of  light  station  load,  only  increases  the 
proportional  cost  B ;  it  therefore  is  desirable  to  discriminate 
against  peak  loads  in  favor  of  day  loads  and  night  loads. 


48  GENERAL  LECTURES 

For  this  purpose,  two-rate  meters  have  been  developed, 
that  is,  meters  which  charge  a  higher  price  for  power 
consumed  during  the  peak  of  the  load  curve,  than  for 
power  consumed  during  the  light  station  loads.  To  even 
out  load  curves,  and  cut  down  the  peak  load,  maximum- 
demand  meters  have  been  developed,  that  is,  meters  which 
charge  for  power  somewhat  in  proportion  to  the  load  factor 
of  the  circuit  controlled  by  the  meter.  Where  the  circuit 
is  a  lighting  circuit,  and  the  maximum  demand  therefore 
coincides  with  the  station  peak,  this  is  effective,  but  on 
other  classes  of  load  the  maximum-demand  meters  may 
discriminate  against  the  station.  For  instance,  a  motor 
load  giving  a  high  maximum  during  some  part  of  the  day, 
and  no  load  during  the  station  peak,  would  be  preferable 
to  the  station  to  a  uniform  load  throughout  the  day,  includ- 
ing the  station  peak,  while  the  maximum-demand  meter 
would  discriminate  against  the  former. 

By  a  careful  development  of  summer  lighting  loads  and 
motor  day  loads,  the  load  factors  of  direct-current  distribu- 
tion systems  have  been  raised  to  very  high  values,  50  to 
60  per  cent. ;  but  in  the  average  alternating-current  system, 
the  failure  of  developing  a  motor  load  frequently  results 
in  very  unsatisfactory  yearly  load  factors. 

The  load  curve  of  a  railway  circuit  is  about  the  shape  of 
that  shown  in  Fig.  1 7 :  a  fairly  steady  load  during  the  day, 
with  a  morning  peak  and  an  evening  peak,  occasionally 
a  smaller  noon  peak  and  a  small  second  peak  later  in  the 
evening,  then  tapering  down  to  a  low  value  during  the  night. 
The  average  load  factor  usually  is  far  higher  than  in  a 
lighting  circuit,  in  Fig.  17:  54.3  per  cent. 

In  defining  the  load  factor,  it  is  necessary  to  state  not 
only  the  time  over  which  the  load  is  to  be  averaged,  as  a 
day,  or  a  year,  but  also  the  length  of  time  which  the  maxi- 


LOAD  FACTOR  AND  COST  OF  POWER 


49 


mum  load  must  last,  to  be  counted.  For  instance,  a 
short-circuit  of  a  large  motor  during  off-peak  load,  which  is 
opened  by  the  blowing  of  the  fuses,  may  momentarily 
carry  the  load  far  beyond  the  station  peak  without  being 
objectional.  The  minimum  duration  of  maximum  load, 
which  is  chosen  in  determining  the  load  factor,  is  that 
which  is  permissible  without  being  objectionable  for  the 
purpose  for  which  the  power  is  distributed.  Thus  in  a 
lighting  system,  where  voltage  regulation  is  of  foremost 


\ 


f0 


12 

X 


I 


FIG.  17. — Railroad  load  curve. 

importance,  minutes  may  be  chosen,  and  maximum  load 
may  be  defined  as  the  average  load  during  that  minute 
during  which  the  load  is  a  maximum;  while  in  a  railway 
system  J^  hour  may  be  used  as  a  duration  of  maximum 
load,  as  a  railway  system  is  not  so  much  affected  by  a  drop 
of  voltage  due  to  overload,  and  an  overload  of  less  than 
J^  hour  may  be  carried  by  the  overload  capacity  of  the 
generators  and  the  heat  storage  of  the  steam  boilers;  so 
that  a  peak  load  requires  serious  consideration  only  when 
it  exceeds  hour. 


50  GENERAL  LECTURES 

Where  several  classes  of  load  are  supplied  by  the  same 
station,  or  even  where  the  power  supply  to  several  distri- 
bution systems  combines  into  one  generating  station,  the 
average  load  factor  of  the  total  load  usually  is  higher  than 
the  average  of  the  load  factors  of  the  individual  components 
of  the  load,  due  to  the  maximum  peaks  of  the  various  loads 
not  coinciding.  In  other  words,  the  maximum  load  of 
the  entire  system  is  less  than  the  sum  of  the  maximum 
loads  of  the  parts  of  the  system.  This  ratio:  sum  of  the 
maximum  loads  of  all  the  parts  of  the  system,  divided  by  the 
maximum  load  of  the  entire  system,  is  called  the  diversity 
factor  of  the  total  load. 

The  greater  the  diversity  factor,  the  higher  obviously 
is  the  load  factor  of  the  total  system,  and  the  higher,  there- 
fore, the  economy,  and  the  most  economical  operation, 
therefore,  is  afforded  by  those  stations,  which  combine 
all  the  power  supply,  for  lighting,  power,  railways,  etc., 
into  one  hugh  unified  system.  Such  systems,  as  exempli- 
fied by  the  Commonwealth  Edison  Company  of  Chicago, 
thus  are  becoming  of  rapidly  increasing  importance  in  the 
electrical  industry. 


FIFTH  LECTURE 
LONG-DISTANCE  TRANSMISSION 

Three-phase  is  used  altogether  for  long-distance  trans- 
mission. Two-phase  is  not  used  any  more,  and  direct 
current  is  being  proposed,  having  been  used  abroad  in  a 
few  cases;  but  due  to  the  difficulty  of  generation  and  utili- 
zation, it  is  not  probable  that  it  will  find  any  extended  use, 
so  that  it  does  not  need  to  be  considered. 

FREQUENCY 

The  frequency  depends  to  a  great  extent  on  the  character 
of  the  load,  that  is,  whether  the  power  is  used  for  alternating- 
current  distribution — 60  cycles — or  for  conversion  to 
direct  current — 25  cycles.  For  the  transmission  line, 
25  cycles  has  the  advantage  that  the  charging  current  is 
less  and  the  inductive  drop  is  less,  because  charging  current 
and  inductance  voltage  are  proportional  to  the  frequency. 
This  advantage,  however,  is  not  so  to  handicap  the  use 
of  60  cycles  even  in  very  long  transmission  lines. 

VOLTAGE 

Eleven  thousand  to  13,000  volts  and  in  a  few  instances 
even  22,000  volts  have  been  used  for  shorter  distances,  as 
10  to  20  miles,  since  this  is  about  the  highest  voltage  for 
which  generators  can  be  built;  its  use,  therefore,  saves 
the  step-up  transformers,  that  is,  the  generator  feeds 
directly  into  the  line  and  to  the  step-down  transformers 
for  the  regular  load.  However,  the  transmission  range  of 
these  voltages  is  so  low,  and  the  design  of  the  high-voltage 

generator  at  such  disadvantage  by  localized  heating  due 

51 


52  GENERAL  LECTURES 

to  the  heavy  insulation  required,  and  by  corona  in  the 
generator  coils,  that  the  general  tendency  of  the  industry 
is  away  from  directly  generating  the  transmission  voltage. 

The  next  step  is  30,000  volts;  that  is,  33,000  volts  at  the 
generator,  30,000  at  the  receiving  end  of  the  line.  No 
intermediate  voltages  between  this  and  the  voltage 
for  which  generators  can  be  wound  is  used,  as  30,000  volts 
does  not  yet  offer  any  insulator  troubles ;  but  line  insulators 
can  be  built  at  moderate  cost  for  this  voltage,  and  as  step-up 
transformers  have  to  be  used,  it  is  not  worth  while  to 
consider  any  lower  voltage  than  33,000  volts.  This  voltage 
transmits  economically  up  to  distances  of  50  to  60  miles. 

Forty  thousand  to  44,000  volts  is  the  next  step:  it  was 
the  highest  transmission  voltage,  at  which  reliable  .opera- 
tion could  be  assured  with  the  former  or  pin  type  of  insula- 
tor: a  few  6o,ooo-volt  systems  were  tried,  but  were  not 
very  successful  regarding  reliability. 

The  development  of  the  suspension  insulator  entirely 
changed  the  situation:  it  made  it  possible  to  insulate 
with  a  very  high  safety  factor  at  moderate  cost  practically 
any  voltage.  The  line  insulator  thereby  vanished  as 
limitation  of  the  voltage  permissible  in  transmission  lines, 
and  lines  of  100,000  volts  and  over  have  become  quite 
frequent,  gave  very  successful  operation,  and  the  only 
voltage  limit  in  transmission  now  is  the  corona  loss  from 
the  line  conductor,  but  not  the  line  insulator. 

For  these  high-voltage  transmissions,  steel-tower  lines 
are  almost  exclusively  used. 

The  cost  of  a  long-distance  transmission  line  depends  on 
the  voltage  used. 

The  cost  of  line  conductors  decreases  with  the  square  of 
the  voltage. 

At  twice  the  voltage,  twice  the  line  drop  can  -be  allowed 


LONG-DISTANCE  TRANSMISSION  53 

with  the  same  loss;  at  twice  the  voltage  the  current  is  only 
half  for  the  same  power,  and  twice  the  drop  with  half  the 
current  gives  four  times  the  resistance,  that  is,  one-quarter 
the  conductor  section  and  cost. 

The  cost  of  line  insulators  increases  with  increase  of 
voltage.  The  cost  of  pole  line  increases  with  increase  of 
voltage,  since  greater  distance  between  the  conductors  is 
necessary  and  so  longer  poles  or  higher  towers,  longer 
cross  arms,  and  heavier  construction,  and  not  so  many 
circuits  can  be  carried  on  the  same  pole  line.  In  general, 
a  good  safe  margin  is  given  by  allowing  i  foot  for  every 
10,000  volts  between  the  conductors. 

The  lower  the  voltage,  the  greater  in  general  is  the  reli- 
ability of  operation,  since  a  larger  margin  of  safety  can  be 
allowed.  However,  the  difference  is  not  great,  and  in 
the  contrary,  extremely  high-voltage  lines  have  shown 
a  considerable  immunity  from  lightning  disturbances,  that 
is,  their  normal  insulation  is  sufficient  to  protect  them  from 
most  lightning  effects. 

Since  a  part  of  the  cost  of  the  transmission  line  decreases, 
another  part  increases  with  the  voltage,  a  certain  voltage 
will  be  most  economical. 

Lower  voltage  increases  the  cost  of  the  conductor,  higher 
voltage  increases  the  cost  of  insulators  and  line  construction, 
and  may  decrease  the  reliability. 

The  most  economical  voltage  of  a  transmission  line  varies 
with  the  cost  of  copper.  When  copper  is  very  high,  higher 
voltages  are  more  economical  than  when  copper  is  low. 
The  same  applies  to  aluminum,  since  the  price  of  aluminum 
has  been  varied  with  that  of  copper. 

Aluminum  generally  is  used  as  stranded  conductor.  In 
the  early  days  single  wire  gave  much  trouble  by  flaws  in  the 
wire.  Aluminum  expands  more  than  copper  with  tempera- 


54  GENERAL  LECTURES 

ture  changes,  and  so  when  installing  the  line  in  summer, 
a  greater  sag  must  be  allowed  than  with  copper,  otherwise  it 
stretches  so  tight  in  winter  that  it  may  tear  apart.  Alumi- 
num also  is  more  difficult  to  join  together,  since  it  cannot 
be  welded. 

For  the  same  conductivity  an  aluminum  line  has  about 
twice  the  size,  but  one-half  of  the  weight  of  a  copper  con- 
ductor, and  costs  a  little  less;  on  the  other  hand,  copper 
has  a  permanent  value,  while  the  price  of  aluminum  may 
sometime  drop  altogether,  as  the  metal  has  no  intrinsic 
value,  being  one  of  the  most  common  constituents  of  the 
surface  of  the  earth,  and  its  cost  is  merely  that  of  its 
separation  or  reduction. 

LOSSES  IN  LINE  DUE  TO  HIGH  VOLTAGE 

The  loss  in  the  line  by  brush  discharge  or  corona  effect 
is  nothing  up  to  a  certain  voltage,  but  at  a  certain  voltage  it 
begins  and  very  rapidly  increases. 

The  voltage  at  which  the  loss  by  corona  begins  in  a  trans- 
mission line  is  where  the  air  at  and  near  the  surface  of  the 
conductor,  and  up  to  a  small  distance  from  the  conductor, 
has  broken  down,  becomes  conducting  and  thus  luminous. 

There  is  thus  a  voltage  e0 — usually  not  far  from  100 
kilo  volts  (i  kilovolt  =  1000  volts)  under  industrial  trans- 
mission-line conditions — at  which  the  breakdown  gradient 
of  air,  21  kilovolts  (alternating  effective)  per  centimeter 
(53,ooo  volts  per  inch)  is  reached  at  the  surface  of  the 
transmission  wire. 

This  voltage,  eot  the  " disruptive  critical  voltage" 
of  the  line,  is  given  by: 

e0  =  84  rd  log  -  kilovolts    alternating    between 

three-phase  lines 
where : 


LONG-DISTANCE  TRANSMISSION  55 

r  =  radius  of  conductor  in  centimeters. 

5  =  spacing  between  conductor  centers  in  centi- 
meters. 

5  =  air  density  factor  =  i  at  25°C.  and  76- 
centimeter  barometer,  thus 

3.926 
5    =^~+~t 

where  t  =  temperature,  in  degrees  C.,  and  b  =  barome- 

ter, in  centimeters  of  mercury. 

(The  log  is  the  common  logarithm.) 

This  applies  only  to  round  conductors  with  smooth 
polished  surface  ;  if  the  surface  of  the  conductor  is  roughened 
or  weathered,  e0  is  about  5  per  cent,  lower. 

If  the  conductor  is  a  seven-strand  cable,  with  r  as  the 
outer  or  overall  radius,  e0  is  about  15  per  cent,  lower. 

However,  no  appreciable  loss  occurs  yet  at  this  voltage, 
e0.  Only  at  a  little  higher  voltage,  ev,  the  "visual  critical 
voltage"  of  the  line,  when  the  breakdown  of  the  air  has 
extended  from  the  wire  a  little  ways,  an  appreciable  loss 
begins,  and  the  conductor  becomes  luminous  in  the  dark. 

The  visual  critical  voltage  is  given  by  : 

I  °'3    1 


While  thus  a  material  loss  begins  only  at  e,,  and  not 
at  e0,  the  amount  of  loss,  where  it  occurs,  is  proportional 
to  the  square  of  the  excess  voltage  over  eot  and  is  given  by 
the  expression: 

p  =  -^  (/  +  25)  J-  (e  -  O2  io-5  kilowatts 

per  kilometer  of  three-phase  line  (three  conductors). 
Where: 

/  =  frequency, 

e  =  operating  voltage  between  three-phase  lines. 


56  GENERAL  LECTURES 

The  corona  loss  thus  increases  with  the  frequency,  and 
very  rapidly  increases  with  the  voltage  and  it  therefore  is 
not  safe  to  materially  exceed  the  voltage  e0  in  transmis- 
sion lines. 

e0  and  ev  depend  on  size  of  conductor,  and  distance  from 
return  conductor,  and  are  proportional  to  the  air  density, 
that  is,  at  higher  temperature  and  lower  barometric 
pressure,  e0  and  ev  are  lower.  Thus  at  an  altitude  where 
the  barometer  reads  24  inches — about  6000  feet  elevation- 
corona  begins  already  at  24/30  =  0.8  times  the  voltage 
at  which  it  begins  at  the  sea  level. 

Thus  lines  traversing  high  altitudes  are  liable  to  be  much 
more  affected  by  corona  losses,  and  the  question  of  altitude 
requires  serious  consideration. 

In  addition  to  the  normal  corona  loss,  there  may  be  very 
material  additional  corona  losses  already  at  lower  voltages, 
under  conditions  of  heavy  rain,  and  especially  snow  storms. 

In  general,  it  may  probably  be  said  that  with  the  sizes 
of  wires,  and  distances  between  wires  usual  in  long-distance 
transmissions,  corona  losses  are  rarely  to  be  feared  at 
line  voltages  below  100,000;  but  at  line  voltages  above  100,- 
ooo,  the  question  of  corona  on  the  transmission  line,  and 
the  possible  amount  of  loss  caused  thereby,  should  be 
investigated. 

In  Fig.  1 8  are  shown  as  ordinates  the  line  voltages 
ev  (voltage  between  three-phase  lines),  at  which  lumi- 
nosity by  corona  begins,  for  different  spacings  of  the  line 
conductors  as  abscissae,  and  for  different  conductor 
diameters,  at  sea  level,  that  is,  air  density  5  =  i. 

In  high-potential  transformers  in  the  coils  usually  no 
corona  effects  occur,  because  the  diameter  of  the  coil  or 
the  thickness  is  large  enough,  but  the  leads  connecting  the 
coils  with  each  other  and  with  the  outside,  if  not  chosen 


LONG-DISTANCE  TRANSMISSION 


57 


very  large  in  diameter,   may  give  corona  effects  and  so 
break  down. 

In  a  line  or  transformer,  if  one  side  is  grounded,  the  other 
side  has  full  voltage  against  ground,  and  so  may  give  corona 


Kv. 


120 


KILOVOLTS  BETWEEN  THREE-PHASE  LINES 


2         4        6        8       10       12      14       16       18      20      22      24      26      28     30      32 

Spacing  between  Wires  (Feet) 
FIG.  1 8. —  Corona  voltage  of  three-phase  lines. 

effects  and  break  down;  while  if  not  grounded,  both  sides 
have  half  voltage  against  ground  and  so  give  no  corona 
effect.  In  the  first  case,  the  line  or  transformer  so  may 


58  GENERAL  LECTURES 

break  down,  although,  the  potential  differences  between 
the  terminals  are  no  greater  than  in  the  second  case. 

For  instance,  in  a  2oo,ooo-volt  transformer  or  line,  from 
each  terminal  to  ground  are  100,000  volts,  and  if  the  con- 
ductor diameter  is  J^-inch,  no  corona  effects  occur.  If 
now  one  terminal  is  grounded,  the  other  terminal  has 
200,000  volts  to  ground  and  so  at  J^-inch  diameter  gives 
corona  effects,  that  is,  glow  and  streamers  which  may 
destroy  the  insulating  material  or  produce  high-frequency 
oscillations. 

At  very  high  voltages,  it  is,  therefore,  necessary  to  have 
the  system  statically  balanced  or  symmetrical,  that  is, 
have  the  same  potential  differences  from  all  the  conductors 
to  the  ground. 

In  circuits  inductively  connected  (that  is  by  transforma- 
tion) to  circuits  of  higher  voltage,  such  static  unbalancing 
of  the  higher  voltage  circuit  may  endanger  the  lower 
voltage  circuit,  especially  if  the  latter  is  isolated  from 
ground. 

Suppose  in  a  ioo,ooo-volt  system,  one  line  grounds. 
The  average  potential  difference  of  this  system,  and  thus 
also  of  the  high-potential  coils  of  the  step-down  trans- 
formers, is  the  Y- voltage,  or  58,000  volts  against  ground. 
Even  if  the  step-down  transformer  is  perfectly  isolated, 
there  is  then  a  path  to  ground,  of  the  58,000  volts  unbal- 
anced voltage,  over  two  capacities  in  series:  the  capacity 
from  the  high-potential  transformer  winding  as  one  con- 
denser plate,  to  the  low-potential  or  secondary  transformer 
winding  as  other  condenser  plate,  and  from  the  low- 
potential  transformer  winding  or  secondary  circuit,  to 
ground.  Depending  on  the  two  capacities,  the  voltages 
thus  divide,  but  if  the  normal  voltage  of  the  secondary 
circuit  is  low,  for  instance  2200  or  6600  volts,  even  a  small 


LONG-DISTANCE  TRANSMISSION  59 

part  of  the  static  voltage  of  58,000,  on  the  condenser  from 
the  secondary  to  ground,  may  be  destructive  by  causing 
continual  spark  discharges,  finally  followed  by  rupture. 

Some  transformer  connections,  as  open  delta,  give  such 
static  unbalancing  even  without  ground  on  the  primary. 

It  follows  herefrom,  that  any  secondary  circuit,  connected 
by  transformation  with  a  primary  circuit  of  much  higher 
voltage,  must  be  protected  against  static  voltages  induced 
by  the  primary  winding  and  comparable  in  magnitude 
with  the  primary  voltage. 

Any  overvoltage  protective  device,  as  lightning  arrester, 
or  a  grounding  of  the  secondary  circuit,  gives  such  protec- 
tion; and  even  a  very  high-resistance  path  to  ground  — 
as  a  high-resistance  rod  with  some  spark  gaps  in  series  or 
so-called  "static  discharger"  —  affords  complete  protection, 
as  the  power,  which  has  to  be  discharged,  is  extremely 
small,  being  due  to  static  induction  through  the  trans- 
former, and  the  only  danger  is  the  disruptive  effect  of  its 
high  voltage. 

Any  electric  circuit,  and  so  also  the  transmission  line, 
contains  inductance  and  capacity,  and,  therefore,  stores 
energy  as  electromagnetic  energy  in  the  magnetic  field  due 
to  the  current,  and  as  electrostatic  energy,  or  electrostatic 
charge,  due  to  the  voltage. 

If: 

e  =  voltage,  C  =  capacity. 
i  =  current,  L  =  inductance. 

the  electrostatic  energy  is: 


2 

and  the  electromagnetic  energy: 

PL 

2 


60  GENERAL  LECTURES 

In  a  high-potential  transmission  line  both  energies  are 
of  about  the  same  magnitude,  and  the  energy  can,  there- 
fore, seesaw  between  the  two  forms  and  thereby  produce 
oscillations  and  surges  resulting  in  the  production  of  high 
voltages,  which  are  not  liable  to  occur  in  circuits  in  which 
one  of  the  forms  of  stored  energy  is  small  compared  with 
the  other. 

In  distribution  systems  up  to  2200  volts  and  even  some- 
what higher,  the  electrostatic  energy  is  still  negligible  and 
only  the  electromagnetic  energy  appreciable. 

In  static  machines  the  electrostatic  energy  is  appreciable, 
but  the  electromagnetic  energy  negligible. 

LINES  AND  TRANSFORMERS 

At  voltages  above  25,000  step-up  and  step-down  trans- 
formers are  always  used,  which  are,  therefore,  a  part  of  the 
high-potential  circuit. 

Three-phase  is  always  used  in  the  transmission  line. 

Some  of  the  available  transformer  connections  are  given 
in  Figs.  19  and  20. 

Grounding  the  neutral  of  the  system  has  the  advantage  of 
maintaining  static  balance  and  so  avoiding  oscillations  and 
disturbances  in  case  of  an  accidental  static  unbalancing,  as 
for  instance,  the  grounding  of  one  line.  It  has  the  dis- 
advantage that  a  ground  on  one  circuit  is  a  short-circuit 
and  so  shuts  down  the  circuit,  while  with  the  ungrounded 
circuit,  the  grounding  of  one  line  merely  produces  a  static 
unbalancing,  which  can  be  taken  care  of  by  protective 
devices  and  larger  margin  in  the  insulators.  The  rela- 
tion between  transmission  lines  with  grounded  neutral  and 
lines  with  ungrounded  neutral  thus  is  essentially  that  be- 
tween cheapness  and  between  reliability:  where  relia- 
bility is  of  foremost  importance  and  justifies  the  some- 


LONG-DISTANCE  TRANSMISSION 


61 


what  higher  cost  of  better  insulation,  the  isolated  delta 
system  of  the  ungrounded  transmission  is  preferable ;  where 


/.) 


oem-r 


3.)  r-r 


.  TWO-PHASE 


FIG.   19. — Transformer  connections. 

cheapness  of  construction  is  of  the  greatest  importance, 
even  at  the  sacrifice  of  some  reliability,  the  grounded  Y- 
system,  that  is,  system  with  grounded  neutral  is  indicated. 


62 


GENERAL  LECTURES 


In  connections  i,  4  and  6  no  neutral  is  available  for 
grounding  and  so  three  separate  transformers  have  to  be 
installed  in  F-connection  for  getting  the  neutral. 

In  connection  2  and  3  the  neutral  can  be  brought  out 
from  the  transformer  neutral. 

In  the  T-connection  5  and  7,  the  neutral  is  brought  out 
from  a  point  at  one-third  of  the  teaser  transformer  winding. 


FIG.   20. — Six-phase  transformer  connections. 

In  general  the  connection  i  is  the  safest  and  therefore 
is  preferable,  as  every  transformer  coil  connects  between 
points  of  fixed  voltage,  the  lines,  and  therefore  no  excess 
voltages  are  liable  to  appear  on  a  transformer  coil,  as 
may  in  cases  where  high-potential  coils  of  different  trans- 
formers are  in  series  with  each  other,  in  the  Y  and  the  T- 
connection,  as  will  be  seen  later. 


LONG-DISTANCE  TRANSMISSION  63 

> 

Assuming  the  line  properly  installed  and  insulated,  break- 
downs may  occur,  either  from  mechanical  accidents  or  by 
high  voltages  appearing  in  the  line. 

HIGH-VOLTAGE  DISTURBANCES  IN  TRANSMISSION  LINES 

These  may  be: 

A.  Of   fundamental   frequency, that  is,    the  same  fre- 
quency as  the  alternating-current  machine  circuit. 

B.  Some  higher  harmonic  of  the  generator  wave,  that  is, 
some  odd  multiple  of  the  generator  frequency. 

C.  Of  frequencies  entirely  independent  of  the  generator, 
or  of  a  frequency  which  originates  in  the  circuit,  that  is, 
high-frequency  oscillations  as  arcing  grounds,  etc. 

If  a  capacity  is  in  series  with  an  inductance,  as  the  line 
capacity  and  the  line  inductance,  the  capacity  reactance 
and  the  inductive  reactance  are  opposed  to  each  other;  if 
they  happened  to  be  equal  they  would  neutralize  each  other, 
the  current  would  depend  on  the  resistance  only  and 
therefore  be  very  large,  and  with  this  very  large  current 
passing  through  the  inductance  and  capacity,  the  voltage 
at  the  inductance  and  at  the  capacity  would  be  very  high. 

For  instance,  if  we  have  20,000  volts  supplied  to  a  circuit 
having  a  resistance  of  10  ohms  and  a  capacity  reactance  of 
1000  ohms,  then  the  total  impedance  of  the  circuit  is 
Vio2  +  iooo2  =  1000  and  the  current  in  the  circuit 

20,000 

-  =  20  amperes, 
iooo 

If  now  in  addition  to  the  10  ohms  resistance  and  iooo 
ohms  capacity  reactance,  the  circuit  contains  iooo  ohms 
inductive  reactance,  the  total  reactance  of  the  circuit  is 
iooo  —  iooo  =  o  ohms,  and  the  impedance  is  the  same  as 

a  a 

the  resistance,  or  10  ohms.     The  current  therefore  -  =  -  = 

2  T 


64  GENERAL  LECTURES 

2000  amperes,  and  the  voltage  at  the  capacity,  therefore,  is: 
capacity  reactance  times  amperes  =  2,000,000  volts,  and 
the  same  voltage  exists  at  the  inductive  reactance. 

These  voltages  are  far  beyond  destruction.  That  is,  if 
in  a  circuit  of  low  resistance  and  high  capacity  reactance,  a 
high  inductive  reactance  is  put  in  series  with  the  capacity 
reactance,  excessive  voltages  are  produced. 

In  a  transmission  line  the  capacity  of  the  line  consumes 
for  instance  10  per  cent,  of  full-load  current;  that  is,  full- 
load  voltage  sends  only  10  per  cent,  of  full-load  current 
through  the  capacity.  To  send  full-load  current  through 
the  capacity  so  would  require  10  times  full-load  voltage. 

With  a  line  reactance  of  20  per  cent.,  20  per  cent,  or  J-£ 
of  full-load  voltage  sends  full-load  current  through  the 
inductive  reactance,  while  10  times  full-load  voltage  is 
required  by  the  capacity  reactance;  the  capacity  reactance, 
therefore,  is  about  50  times  larger  than  the  inductive  re- 
actance at  the  generator  frequency  and,  therefore,  cannot 
build  up  with  it  to  excessive  voltages;  but  to  get  resonance 
with  the  fundamental  frequency  requires  an  inductive 
reactance  about  50  times  greater  than  the  line  reactance. 

The  only  reactance  in  the  system  which  is  large  enough 
to  build  up  with  the  capacity  reactance  is  the  open-circuit 
reactance  of  the  transformers.  This  is  of  about  the  same 
size  as  the  capacity  reactance,  since  a  transformer  at  open 
circuit  and  full  voltage  takes  about  10  per  cent,  of  full- 
load  current,  and  the  capacity  reactance  also  takes  about 
10  per  cent,  of  full-load  current,  in  moderately  short  lines. 

If,  therefore,  a  high-potential  coil  of  a  transformer  at 
open  secondary  circuit  is  connected  in  series  with  a  trans- 
mission line,  destructive  voltages  may  be  produced,  by 
the  reactance  of  the  transformer  building  up  with  the  line 
capacity.  In  those  transformer  connections  in  which 


LONG-DISTANCE  TRANSMISSION 


65 


several  high-potential  coils  of  different  transformers  are 
connected  between  the  transmission  wires,  this  may  occur 
if  the  low-tension  coil  of  one  of  the  transformers  accidentally 
opens  and  the  high-potential  coil  of  this  transformer  then 
acts  as  inductive  reactance  in  series  with  the  line  capacity 
in  the  circuit  of  the  other  transformer. 


0 

J. V       \ 

^vy» 


Fie. 


-Fundamental  frequency  resonance. 


This  may  occur  for  instance  in  transformer  connection  2, 
Fig.  19,  if  as  shown  in  Fig  21,  the  low-tension  coil  c  opens. 
Then  the  high-tension  coil  C  is  an  inductive  reactance  in 
series  with  the  line  capacity  from  3  to  i,  energized  by 
transformer  A\  and  C  is  a  high  inductive  reactance  in 
series  with  the  line  capacity  from  3  to  2  in  a  circuit  of 


c 


FIG.  22. — Fundamental  frequency  resonance. 

voltage  B.  That  is,  from  3  to  i  and  from  3  to  2  excessive 
voltages  are  produced.  So  also  in  7-connection,  Fig.  22, 
if  for  instance  the  low- tension  coil  a  opens,  the  correspond- 
ing high-tension  coil  A  is  a  high  inductive  reactance  in 
series  with  the  line  capacities  in  a  circuit  of  the  voltages 
of  the  two  halves,  B  and  C,  of  the  other  transformer,  and 


66  GENERAL  LECTURES 

excessive  voltages,  therefore,  appear  from  i  to  2  and  from 
i  to  3. 

This  danger  of  excessive  voltages  by  the  accidental  open- 
ing of  a  transformer  low-tension  coil  does  not  exist  in  delta- 
connection,  since  in  this  always  only  one  transformer 
connects  from  line  to  line.  It  is  greatly  reduced  since  the 
use  of  triple-pole  switches  became  general;  and  is  very 
much  less  where  several  sets  of  transformers  are  used 'in 
multiple,  since  even  if  in  one  set  a  low-tension  coil  opens, 
the  other  sets  maintain  the  voltage  triangle. 

Especially  dangerous  in  this  respect,  therefore,  is  the  L- 
connection  No.  6 ;  since  in  this  case,  when  using  two  trans- 
formers in  open  delta,  for  smaller  systems  only  one  set  is 
installed  and  an  accident  to  one  of  the  transformers  causes 
excessive  voltages  between  its  line  and  the  two  other  lines. 

The  open-circuit  reactance  of  the  transformer  is  the 
only  reactance  high  enough  to  give  destructive  voltages 
at  generator  frequency,  and  in  high-potential  disturbances, 
the  transformer  connections  should  first  be  carefully  in- 
vestigated to  see  whether  this  has  occurred. 

However,  a  considerable  and  destructive,  voltage  rise 
of  fundamental  frequency  may  occur  by  the  combination 
of  a  partial  resonance  rise,  as  discussed  above,  with  over- 
excitation  of  the  generators  and  increase  of  speed  by  the 
racing  of  the  turbines.  For  instance, '  consider  a  very 
long  high- voltage  transmission  line — of  150  to  200  miles 
and  110,000  to  150,000  volts,  thus  a  capacity  current  of 
50  per  cent,  or  more  of  full-load  current.  Suppose  now  a 
short-circuit  of  appreciable  resistance,  that  is,  high  power 
consumption,  occurs  at  the  end  of  the  lines:  to  maintain 
the  voltage,  the  automatic  regulators  on  the  generators 
increase  the  field  excitation  to  the  maximum.  To  main- 
tain the  speed,  the  turbine  governors  open  the  gates  wide. 


LONG-DISTANCE  TRANSMISSION  67 

Suppose  now  the  short-circuit  is  suddenly  opened:  before 
the  water  gates  can  shut  off,  the  turbines  may  have  in- 
creased 50  per  cent,  in  speed;  the  field  excitation  would 
give  a  voltage  far  above  normal  at  normal  speed,  and 
still  50  per  cent,  higher  at  the  speed  of  the  racing  turbines, 
and  adding  thereto  the  voltage  rise  in  the  line  capacity, 
it  follows  that  under  such  conditions  a  voltage  rise,  at 
normal  frequency  and  backed  by  the  machine  power, 
may  occur  of  100  per  cent,  or  more.  Such  systems  thus 
make  voltage  control  by  synchronous  condensers  at  the 
receiving  end  desirable,  if  not  necessary. 


SIXTH  LECTURE 
HIGHER  HARMONICS  OF  THE  GENERATOR  WAVE 

The  open-circuit  reactance  of  the  transformer  is  the 
only  reactance  high  enough  to  give  resonance  with  the  line 
capacity  at  fundamental  frequency. 

All  other  reactances  are  too  low  for  this. 

Since,  however,  the  inductive  reactance  increases  and  the 
capacity  reactance  decreases  proportionally  to  the  fre- 
quency, the  two  reactances  come  nearer  together  for  higher 
frequency,  that  is,  for  the  higher  harmonics  of  the  generator 
wave;  and  for  some  of  the  higher  harmonics  of  the  genera- 
tor wave  resonance  rise  of  voltage  so  may  occur  between 
the  line  capacity  and  the  circuit  inductance. 

The  origin  and  existence  of  higher  harmonics,  there- 
fore, bears  investigation  in  transformers,  transmission 
lines  and  cable  systems. 

ORIGIN  OF  HIGHER  HARMONICS 

Higher  harmonics  may  originate  in  synchronous  ma- 
chines, as  generators,  synchronous  motors  and  converters, 
and  in  transformers. 

These  two  classes  of  higher  harmonics  are  very  different. 
The  former  have  constant-potential  character;  the  latter, 
constant-current  character;  their  cure  and  prevention, 
therefore,  must  be  different,  and  the  method  of  elimination 
of  one  may  be  very  harmful  with  the  other  type  of  har- 
monics. For  instance,  the  voltage  produced  by  a  con- 
stant-current harmonic  as  coming  from  a  transformer  is 

eliminated  by  short-circuit,  as  produced  by  delta  connec- 

68 


HIGHER  HARMONICS  OF  THE  GENERA  TOR  WA  VE      69 

tion  on  one  side  of  the  transformer.  Short-circuiting  a 
generator  harmonic,  however,  gives  large  short-circuit 
currents,  due  to  the  constant-potential  character,  and  is 
therefore  dangerous. 

HIGHER  HARMONICS  OF  SYNCHRONOUS  MACHINES 

In  synchronous  machines,  as  alternating-current  genera- 
tors, the  higher  harmonics  are: 

At  No  Load. — First. — The  distribution  of  magnetism 
in  the  air  gap  depends  on  the  shape  of  the  field  poles;  it  is 
not  a  sine  wave;  neither  is  the  e.m.f.  induced  by  it  in  an 
armature  conductor  a  sine  wave. 

Since  there  are  a  number  of  conductors  in  series  on  the 
armature,  the  voltage  wave  is  more  evened  out  than  that  of 
a  single  conductor;  but  still  it  is  not  a  sine  wave,  that  is, 
contains  harmonics  of  which  the  third  is  the  lowest. 

Second. — The  change  of  magnetic  flux  by  the  passage  of 
open  armature  slots  over  the  field  pole  produces  harmonics 
of  e.m.f. ;  that  is,  when  a  large  open  armature  slot  stands  in 
front  of  the  field  pole,  the  magnetic  reluctance  is  high ;  the 
magnetism  is  lower  than  when  no  slot  is  in  front  of  the 
field  pole;  that  is,  by  the  passage  of  the  armature  slots 
the  field  magnetism  pulsates,  the  more  so  the  larger  the 
slots  and  the  fewer  they  are. 

If  there  are  n  slots  per  pole,  this  produces  the  two  har- 
monics 2n  —  i  and  2n  -\-  i. 

At  Load. — Third. — The  armature  reaction  of  a  single-phase 
machine  pulsates  between  zero  at  zero  current  and  a  maxi- 
mum at  maximum  current. 

The  resultant  armature  reaction  of  a  polyphase  machine 
is  constant,  but  locally  there  is  a  pulsation  making  as  many 
cycles  per  pole  as  there  are  phases. 

Since  the  field  magnetism  under  load  is  due  to  the  com- 


70  GENERAL  LECTURES 

bination  of  field  excitation  and  armature  reaction,  the  pulsa- 
tion of  armature  reaction  therefore  causes  a  pulsation  of 
field  magnetism,  and  thereby  higher  harmonics  of  the 
e.m.f.  wave. 

If  m  —  number  of  phases,  the  higher  harmonics :  2m  —  i 
and  2m  +  i  are  produced. 

Fourth. — The  terminal  voltage  under  load  is  the  resultant 
of  the  induced  e.m.f.  and  the  e.m.f.  consumed  by  the 
reactance  of  the  armature  circuit;  that  is,  the  reactance 
produced  by  the  magnetic  flux  produced  by  the  armature 
current  in  the  armature  iron.  This  armature  reactance 
is  not  constant,  but  periodically  varies,  more  or  less,  with 
double  frequency;  that  is,  when  the  armature  coil  is  in 
front  of  the  field  pole  its  magnetic  circuit  is  different  than 
when  it  is  between  the  field  poles,  and  the  reactance 
therefore  is  different. 

This  pulsation  of  armature  reactance  produces  the  third 
harmonic,  since  it  is  of  double  frequency. 

The  most  common  and  prominent  harmonic  so  is  the 
third  harmonic  in  a  synchronous  machine. 

These  harmonics  of  synchronous  machines  are  induced 
e.m.fs.,  that  is,  constant-potential  or  approximately  so. 

HIGHER  HARMONICS  OF  TRANSFORMERS 

In  a  transformer  the  wave  of  e.m.f.  depends  on  that  of 
the  magnetism  and  vice  versa.  That  is,  with  a  sine  wave  of 
e.m.f.,  the  magnetism  must  also  be  a  sine  wave,  and  if  the 
magnetism  is  not  a  sine  wave,  but  contains  higher  har- 
monics, the  e.m.f.  is  not  a  sine  wave,  both  contains  the 
harmonics  induced  by  the  harmonics  of  magnetism. 

The  exciting  current  of  the  transformer  depends  on  the 
magnetism  by  the  hysteresis  cycle;  if  the  magnetism  is  a 
sine  wave,  the  exciting  current,  therefore,  cannot  be  a  sine 


HIGHER  HARMONICS  OF  THE  GENERA  TOR  WAVE      71 

wave,  but  must  contain  higher  harmonics — mainly  the 
third  harmonic,  which  reaches  20  to  30  per  cent,  of  the 
fundamental,  or  even  more  at  saturation. 

In  a  transformer,  e.m.f.  and  exciting  current,  therefore, 
cannot  both  be  sine  waves,  but  a  sine  wave  of  e.m.f.  requires 
an  exciting  current  containing  a  third  harmonic;  and  a 
sine  wave  of  exciting  current  in  a  transformer  or  reactive 
coil  thus  produces  a  third  harmonic  of  e.m.f. 

If,  therefore,  in  a  transformer  the  third  harmonic  is  sup- 
pressed, and  if  this  third  harmonic  should  have  been  20 
per  cent,  of  the  fundamental,  then  its  suppression  produces 
a  third  harmonic  of  magnetism  of  20  per  cent,  in  the  oppo- 
site direction.  A  third  harmonic  of  magnetism,  however^ 
of  20  per  cent.,  induces  a  third  harmonic  of  e.m.f.  of  3  X  20 
=  60  per  cent. ;  the  e.m.f.  being  proportional  to  magnetism 
and  frequency. 

In  three-phase  transformers,  this,  however,  is  the  case 
only  if  the  magnetic  circuit  is  arranged  so  that  the  flux  of 
each  phase  closes  its  circuit  without  passing  through 
another  phase,  so-called  shell-type  transformers.  If,  how- 
ever, the  magnetic  fluxes  of  the  three-phase  transformer 
are  interlinked  so  that  each  magnetic  circuit  interlinks 
with  two  phases — so-called  core-type  transformers — no 
triple-frequency  flux  can  exist  in  a  closed  magnetic  circuit, 
since  the  phases  neutralize,  and  the  triple-frequency  fluxes 
have  open  magnetic  circuits,  thus  usually  are  very  small, 
and  the  triple-frequency  voltages  thus  negligible. 

The  third  harmonic  of  exciting  current  is  positive  at  the 
maximum  of  magnetism,  and  the  third  harmonic  of  magnet- 
ism is  negative  at  the  maximum,  hence  is  zero  and  rising 
at  the  zero  of  the  magnetism;  and  at  this  moment  the  e.m.f. 
induced  by  the  third  harmonic  and  by  the  fundamental, 
therefore,  are  both  maxima  and  in  the  same  direction, 


72 


GENERAL  LECTURES 


that  is,  add.  The  suppression  of  the  third  harmonic 
of  exciting  current  thus  produces  a  very  high  third  harmonic 
of  e.m.f.,  which  greatly  increases  the  maximum  e.m.f. ; 
that  is,  the  e.m.f.  wave  is  very  low  for  a  large  part  of  the 
cycle  and  then  rises  to  a  very  high  peak,  as  shown  in  Fig.  23 ; 
and  the  maximum  e.m.f.  may  exceed  that  of  a  sine  wave  by 
50  per  cent,  and  more,  thus  giving  high  insulation  stress 
and  the  possibility  of  resonance  voltages. 


FIG.  23. — Wave  distortion  by  suppression  of  third  harmonic  in  transformer. 

EFFECTS  OF  HIGHER  HARMONICS 

In  a  three-phase  system  the  three  phases  are  120°  apart, 
and  their  third  harmonics  are  3  X  120°  =  360°  apart,  that 
is,  in  phase  with  each,  and  for  the  third  harmonic  the  three- 
phase  system  therefore  is  a  single-phase  system. 

In  a  balanced  three-phase  system,  the  third  harmonics 
cannot  exist  in  the  voltages  between  the  lines  and  in  the 
line  currents,  if  there  is  no  return  over  the  neutral.  The 


HIGHER  HARMONICS  OF  THE  GENERA  TOR  WA  VE      73 

three  voltages  between  lines,  from  i  to  2,  2  to  3,  and  3  to  i, 
must  add  up  to  zero ;  but  since  the  third  harmonics  would  be 
in  phase  with  each  other,  they  would  not  add  up  to  zero, 
therefore  they  cannot  exist.  The  three  currents,  if  there 
is  no  return  over  the  neutral  or  the  ground,  must  add  up 
to  zero;  and  since  their  third  harmonics  must  be  in  phase 
with  each  other,  they  must  be  absent.  In  a  balanced 
three-phase  system,  third  harmonics  can  exist  only  in  the 
voltage  from  line  to  neutral  or  F-voltage,  in  the  current 
from  line  to  line  or  delta  current,  and  in  the  line  current 
only  if  there  is  a  neutral  return  or  ground  return  to  the 
generator  neutral  or  transformer  neutral. 

In  a  three-phase  generator,  if  the  e.m.f.  of  one  phase  con- 
tains a  third  harmonic,  as  is  usually  the  case,  then  by  con- 
necting the  three  phases  in  delta  connection,  the  third 
harmonics  of  the  generator  e.m.fs.  are  short-circuited  and 
so  produce  a  triple-frequency  current  circulating  in  the 
generator  delta.  This  triple-frequency  circulating  current 
can  be  measured  by  connecting  an  ammeter  in  one  corner  of 
the  generator  delta,  and  the  sum  of  voltages  of  the  three 
third  harmonics  can  be  measured  by  putting  a  voltmeter 
in  a  corner  of  the  generator  delta.  This  local  current  in  the 
generator  winding  is  the  triple-frequency  voltage  divided 
by  the  generator  impedance  (the  stationary  impedance, 
at  triple  frequency,  but  not  the  synchronous  impedance, 
since  the  latter  includes  armature  reaction) .  In  generators 
of  low  impedance  or  close  regulation,  as  turbine  alternators, 
this  local  current  may  be  far  more  than  full-load  current; 
delta  connection  of  generator  windings  therefore  is  unsafe. 
As  a  result,  generator  windings  are  almost  always  connected 
in  Y.  Even  with  delta  connection  of  generator  windings 
no  triple  frequency  appears  at  the  terminals,  since  its  voltage 
disappears  by  short-circuit. 


74  GENERAL  LECTURES 

If  the  generator  winding  is  connected  in  Y,  the  triple- 
frequency  voltages  from  terminal  to  neutral  are  in  phase 
with  each  other;  that  is,  in  a  three-phase  F-connected 
generator,  a  single-phase  voltage  of  triple  frequency  exists 
between  the  neutral  and  all  three  terminals,  and  the  neutral 
therefore  is  not  a  true  neutral.  Between  the  lines  no  triple- 
frequency  voltage  exists,  since  from  terminal  to  neutral 
and  from  neutral  to  the  other  terminal  the  two  third  har- 
monics are  in  opposition  and  so  neutralize. 

This  third  harmonic  between  generator  neutral  and  line 
must  be  kept  in  mind,  since  when  large  it  may  produce 
dangerous  voltages  by  resonance  with  the  line  capacity. 

When  the  generator  neutral  is  grounded,  the  potential 
difference  from  line  to  ground  is  not  line  voltage  divided  by 
A/3,  that  is,  the  true  Y- voltage  of  the  system;  but  superim- 
posed upon  it  is  this  single-phase  triple-frequency  voltage; 
and  the  voltage  from  line  to  ground,  especially  its  maximum, 
may  be  greatly  increased,  thus  increasing  the  insulation 
strain.  For  this  single-phase  voltage  all  three  lines  go 
together,  and  so  may  cause  static  induction  on  other 
circuits,  as  telephone  lines.  A  circuit  of  this  single-phase 
triple-frequency  voltage  then  exists  from  the  generator 
neutral  over  the  inductance  of  all  three  generator  circuits 
in  multiple,  and  over  the  capacity  of  all  three  lines  to 
ground,  back  to  the  generator  neutral;  that  is,  we  have 
capacity  and  inductance  in  series  in  a  circuit  of  the  triple 
harmonic,  and  if  capacity  and  inductance  are  high  enough, 
we  may  get  a  dangerous  voltage  rise. 

In  this  case  of  grounded  generator  neutral,  if  the  primary 
neutral  of  the  F-connected  step-down  transformers  is 
grounded  also,  and  the  low-tension  side  of  these  trans- 
formers connected  in  Y,  the  third  harmonic  of  the  generator 
has  no  path;  the  current  produced  by  it  would  have  to 


HIGHER  HARMONICS  OF  THE  GENERA  TOR  WA  VE      75 

return  over  the  open-circuit  reactance  of  the  step-down 
transformer,  and  is  limited  thereby  to  a  negligible  value. 

If,  however,  the  secondaries  of  the  step-down  trans- 
formers are  connected  in  delta,  so  that  the  third  harmonic 
can  circulate  in  the  secondary  delta,  the  third  harmonic 
can  flow  through  the  transformer  primary  by  inducing  an 
opposite  current  in  the  secondary;  in  this  case  the  step- 
down  transformer  short-circuits  the  third  harmonic  of  the 
generator.  Grounding  the  primary  neutral  of  step-down 
transformers  with  grounded  generator  neutral,  therefore, 
is  permissible  only  if  the  transformer  secondaries  are  also 
connected  in  Y.  With  delta-connected  transformer  secon- 
daries, however,  it  is  not  safe  to  ground  the  generator 
neutral  and  transformer  neutral;  since  this  produces  a 
triple-frequency  current  in  generator,  line  and  transformer ; 
and  even  if  the  generator  reactance  is  so  high  that  the 
generator  is  not  harmed  by  this  current,  it  may  burn  out 
the  transformer,  and  probably  will  do  so  if  the  transformer 
is  small  compared  with  the  generator. 

This,  therefore,  is  a  case  where  delta  connection  of  the 
transformer  secondaries  does  not  eliminate  the  trouble 
from  the  third  harmonic,  but  makes  it  worse. 

In  other  words,  delta  connection  of  at  least  one  side  of 
the  transformer  eliminates  the  third  harmonic  resulting 
from  the  transformer,  but  is  not  safe  if  it  lets  the  third 
harmonic  of  the  generator  flow. 

The  triple-frequency  voltage  from  line  to  ground  would 
be  eliminated  by  short-circuiting  it  in  this  manner,  by  Y- 
delta  connection  of  step-down  transformer  with  grounded 
generator  and  transformer  neutral,  and  static  induction  on 
other  circuits  so  would  disappear;  but  we  get  magnetic 
induction  from  the  three  triple-frequency  single-phase 
currents  which  now  flow  over  the  lines  to  the  ground. 


76  GENERAL  LECTURES 

If  the  generator  neutral  is  not  grounded,  it  is  safe  to 
ground  transformer  neutrals.  With  ungrounded  generator 
neutral,  a  triple-frequency  voltage  can  be  measured  by 
voltmeter,  which  then  appears  between  generator  neutral 
and  ground;  this  voltage  under  unfavorable  conditions, 
may  give  insulation  strains  in  the  generator  by  resonance 
rise  in  the  circuit  from  generator  neutral  over  triple-fre- 
quency voltage,  generator  inductance,  capacity  from  line 
to  ground  and  capacity  from  ground  to  generator  winding 
in  series. 

In  this  case  the  capacity  is  much  lower  and  the  power 
therefore  much  less,  that  is,  less  danger  exists. 

When  running  two  or  more  three-phase  generators  in 
parallel,  with  grounded  neutrals: 

(a)  If  the  generators  have   different   third  harmonics, 
these    harmonics    are    short-circuited    from    neutral    over 
generator  to  the  other  generator  and  back  to  neutral;  a 
triple-frequency  current  thus  flows  between  the  generators, 
that  is,  the  current  between  the  generators  can  never  be 
made  to  disappear. 

That  is,  for  the  third  harmonic,  the  two  generators  are 
two  single-phase  machines  of  different  voltage,  having  the 
neutral  as  one  terminal  and  the  three  three-phase  terminals 
as  the  other  single-phase  terminal. 

(b)  With  two  identical  generators  running  in  multiple,  if 
the  excitation  is  identically  the  same,   no  current  flows 
between  the  grounded  neutrals.     If  the  excitation  of  the 
two  generators  is  different,  one  is  overexcited  the  other  is 
underexcited  (that  is,  one  carries  leading,  the  other  lagging 
current )  then  a  triple-frequency  current  flows  between  the 
neutrals  of  identical  generators.     Since  in  parallel  operation 
the  terminal  voltages  are  in  phase,  if  by  difference  of  excita- 
tion the  two  terminal  voltages  have  a  different  lag  behind 


HIGHER  HARMONICS  OF  THE  GENERA  TOR  WA  VE      77 

the  induced  e.m.fs.,  the  third  harmonics,  which  lag  three 
times  as  much  as  the  fundamentals,  cannot  be  in  phase  in 
the  two  machines;  and  thus  triple-frequency  current  flows 
between  the  machines. 

In  machines  of  very  low  reactance  as  turbo-alternators, 
even  small  differences  in  excitation  of  identical  machines 
with  grounded  neutral  may  sometimes  cause  very  large 
neutral  currents. 

In  parallel  operation  of  three-phase  machines  with 
grounded  neutral,  machines  of  different  wave  shapes  fre- 
quently cannot  be  run  together  at  all  without  excessive 
neutral  currents,  and  the  ground  has  to  be  taken  off  of  one 
of  the  machine  types. 

Even  with  identical  machines,  care  has  to  be  taken  in 
keeping  the  same  excitation  so  that  it  is  frequently  un- 
desirable to  ground  all  the  neutrals,  but  only  the  neutral 
of  one  machine  is  grounded  and  the  other  machine  neutrals 
are  left  isolated.  In  this  case,  provisions  must  be  made  to 
ground  the  neutral  of  some  other  machine,  if  the  first  one 
is  out  of  service.  The  best  way  is,  when  grounding 
generator  neutrals,  to  ground  through  a  separate  resistance 
for  every  generator  and  to  choose  this  resistance  so  high  as 
to  limit  the  neutral  current,  but  still  low  enough  so  that  in 
case  of  a  ground  on  one  phase,  enough  current  flows  over 
the  neutral  to  open  the  circuit-breaker  of  the  grounded 
phase. 

The  use  of  a  resistance  in  the  generator  neutral  is  very 
desirable  also,  since  it  eliminates  the  danger  of  a  high- 
frequency  oscillation  between  line  and  ground  through  the 
generator  reactance  in  the  path  of  the  third  harmonic,  by 
damping  the  oscillation  in  the  resistance.  For  this  reason, 
the  resistance  should  be  non-inductive.  To  ground  the 
generator  neutral  through  a  reactance  is  very  dangerous 


78  GENERAL  LECTURES 

since   it   intensifies    the    danger   of   a  resonance    voltage 
rise. 

In  grounding  the  generator  neutral,  special  care  is  neces- 
sary to  get  perfect  contact,  since  an  arc  or  loose  contact 
would  generate  a  high  frequency  in  the  circuit  of  the  third 
harmonic  and  so  may  lead  to  a  higher-frequency  oscilla- 
tion between  line  and  ground. 


SEVENTH  LECTURE 

HIGH  FREQUENCY  OSCILLATIONS,  SURGES 
AND  IMPULSES 

In  an  electric  circuit,  in  addition  to  the  power  consump- 
tion by  the  resistance  of  the  lines,  an  energy  storage 
occurs  as  electrostatic  energy,  or  electrostatic  charge  due  to 
the  voltage  on  the  line  (capacity)  ;  and  as  electromagnetic 
energy,  or  magnetic  field  of  the  current  in  the  line  (induc- 
tance). In  the  long-distance  transmission  line,  both 
amounts  of  stored  energy  are  very  considerable,  and  of 
about  equal  magnitude;  the  former  varying  with  the  volt- 
age, the  latter  with  the  current  in  the  line.  Any  change  of 
the  voltage  on  the  line,  or  the  current  in  the  line,  or  the 
relation  between  voltage  and  current,  therefore  requires  a 
corresponding  change  of  the  stored  energy;  that  is,  a  re- 
adjustment of  the  stored  energy  in  the  system,  the  electro- 


static  energy  —  •  and  the  electromagnetic  energy  -  —  •»  from 

that  previous  to  the  changed  circuit  conditions.  This 
readjustment  occurs  by  an  oscillation,  that  is,  a  series  of 
waves  of  voltage  and  of  current,  which  gradually  decreases 
in  intensity,  that  is,  dies  out.  Very  frequently  such  an 
oscillation  or  stationary  electric  wave  is  preceded  by  an 
impulse  or  traveling  wave,  which  rushes  along  the  circuit 
from  the  source  of  disturbance,  and  by  reflection  from  the 
end  of  the  circuit,  etc.,  develops  into  the  oscillation.  Such 
impulses  or  traveling  waves  if  of  very  high  frequency  may 
pass  over  the  circuit  without  resulting  in  an  appreciable 
oscillation,  and  may  not  even  be  periodic,  but  single  uni- 

directional impulses. 

79 


80  GENERAL  LECTURES 

These  oscillating  voltages  and  currents  are  the  result  of 
the  readjustment  of  the  stored  energy  of  the  circuit  to  a 
sudden  change  of  conditions,  and  are  dependent  upon  the 
stored  energy  of  the  circuit,  but  not  upon  the  generator  fre- 
quency or  wave  shape;  therefore  they  occur  in  the  same 
manner,  and  are  of  the  same  frequency,  in  a  25-cycle  sys- 
tem as  in  a  6o-cycle  system,  or  a  high-potential  direct- 
current  transmission;  and  occur  with  sine  waves  of  genera- 
tor voltage  equally  as  with  distorted  generator  waves. 
While  the  power  of  these  oscillations  ultimately  comes  from 
the  generators,  it  is  not  the  generator  wave  nor  one  of  its 
harmonics  which  builds  up,  as  discussed  in  the  previous 
lectures;  but  the  generator  merely  supplies  the  energy, 
which  is  stored  as  electrostatic  charge  of  the  capacity  and 
as  magnetic  field  of  the  inductance,  and  the  readjustment 
of  this  stored  energy  to  the  change  of  circuit  conditions  then 
gives  the  oscillation. 

These  oscillating  voltages  and  currents,  adding  to  the 
generator  voltage  and  current,  thus  increase  the  voltage 
and  the  current  the  more,  the  greater  the  intensity  of  the 
oscillation,  and  so  may  lead  to  destructive  voltages. 

Obviously,  the  intensity  of  the  oscillation,  that  is,  its 
voltage  and  current,  are  the  greater,  the  greater  or  more 
abrupt  the  change  was  in  the  circuit,  which  caused  the  os- 
cillation by  requiring  a  readjustment  of  the  energy  storage. 
The  greatest  change  in  a  circuit,  however,  is  the  change 
from  short-circuit  to  open  circuit,  and  the  instantaneous 
opening  of  a  short-circuit  on  a  transmission  line — as  it 
occasionally  occurs  by  the  sudden  rupture  of  a  short-cir- 
cuiting arc — therefore  gives  rise  to  the  most  powerful,  and 
thereby  most  destructive  oscillation. 

The  wave  length  of  oscillation  thus  depends  on  the 
length  of  the  circuit  in  which  the  stored  energy  readjusts 


HIGH  FREQUENCY  OSCILLATIONS  81 

itself.  For  instance,  in  the  short-circuit  oscillation  of  the 
system,  the  wave  extends  over  the  entire  circuit,  including 
generators  and  transformers;  and  the  entire  circuit  so 
epresents  one  wave,  or  one-half  wave,  that  is,  the  wave- 
length is  very  considerable.  If  the  readjustment  of  stored 
energy  takes  place  only  over  a  section  of  the  circuit,  the 
wave  length  is  shorter.  For  instance,  if  by  a  thunder 
cloud  a  static  charge  is  induced  on  the  transmission  line, 
and  by  a  lightning  flash  in  the  cloud,  the  cloud  discharges, 
the  electrostatic  charge  induced  by  it  on  the  line  is  set  free 
and  dissipates  by  an  oscillation.  In  this  case,  the  length  of 
section  on  which  an  abnormal  charge  existed — i  mile  for  in- 
stance— is  a  half  wave  of  the  oscillation,  and  the  complete 
wave  length  would  thus  be  2  miles.  Or,  if  a  momentary 
discharge  occurs  over  a  lightning  arrester  to  ground,  the 
wave  length  may  be  only  a  few  feet. 

The  velocity  with  which  the  electric  wave  travels  in  an 
overhead  line  is  practically  the  velocity  of  light,  or  about 
188,000  miles  per  second :  it  would  be  exactly  the  velocity  of 
light,  except  that  by  the  resistance  of  the  line  conductor  the 
velocity  is  very  slightly  reduced.  In  an  underground  cable, 
by  the  high  capacity  of  the  cable  insulation,  the  velocity  of 
wave  travel  is  greatly  reduced,  to  about  50  to  70  per  cent, 
of  that  of  light. 

From  the  wave  length  and  the  velocity  follows  the  dura- 
tion or  time  of  one  wave,  and  thereby  the  frequency  of  the 
oscillation.  For  instance,  in  the  wave  of  2  miles'  length  re- 
sulting from  induction  by  a  thunder  cloud,  as  discussed 
above,  the  duration  of  the  wave,  or  the  time  it  takes  to 
travel  the  wave  length  of  2  miles,  at  188,000  miles  per  sec- 
ond velocity,  is  —^ —  -  =  -  -  second,  and  thus,  during 
188,000  94,000 

i  second,  94,000  waves  would  pass,  that  is,  the  frequency  is 


82  GENERAL  LECTURES 

94,000  cycles.  Or,,  if  a  transmission  line  of  80  miles' 
length  short-circuits  at  one  end,  and  then  disconnects  at 
the  other  end  by  the  opening  of  the  circuit-breaker,  in  the 
oscillation  produced  thereby  the  circuit  is  one-half  wave. 
As  the  length  of  the  circuit  is  2  X  80  =  160  miles — conduc- 
tor and  return  conductor — the  half  wave  is  160  miles;  the 
complete  wave  therefore  is  2  X  160  =  320  miles  long,  and 

320  i 

the  duration  of  the  wave  is  — ~ =  —^-   second;   the 

010,000       507 

frequency  587  cycles,  and  if  this  short-circuit  oscillation 
extends  into,  and  includes  the  generating  system,  the  fre- 
quency may  be  still  lower. 

Again,  an  oscillation  of  a  very  short  section  of  the  line,  as 

for  instance,    100  feet  =  - — r-  =  — ~  mile    wave  '  length, 

5200       52.0 


would  have  a  duration  of  the  wave  of 


52.8  X  188,000 
second,  or  a  frequency  of  9.9  millions  of  cycles  per 


9,900,000 
second. 

Hence  the  frequency  of  such  oscillations,  caused  by  the 
readjustment  of  the  stored  energy  of  the  system,  may  vary 
from  values  below  machine  frequency,  up  to  many  mil- 
lions of  cycles  per  second.  It  is  the  higher,  the  shorter  the 
section  of  the  circuit  is  in  which  the  readjustment  of  energy 
occurs.  The  higher  the  frequency,  and  therefore  the  shorter 
the  section  of  the  circuit  in  which  energy  readjustment 
occurs,  obviously  the  less  is  the  amount  of  energy  which  is 
available  in  the  oscillation — the  stored  energy  of  this  sec- 
tion— and  the  less  destructive  therefore  is  the  oscillation. 
That  is,  very  high-frequency  oscillations  are  of  very  low 
energy  and  therefore  of  little  destructiveness ;  but  the  energy 
and  thus  the  destructiveness  of  an  oscillation  increases  with 


HIGH  FREQUENCY  OSCILLATIONS  83 

decreasing  frequency,  and  consequent  increasing  extent  of 
the  oscillation. 

Such  oscillations  in  a  transmission  line  may  result: 

(a)  From  outside  sources,  atmospheric  electric  disturb- 
ances, as  illustrated  in  the  above  instance. 

(b)  They  occur  during  normal  operation  of  the  system: 
any  change  of  load,  or  switching  operation,  as  connecting  or 
disconnecting  circuits,  etc.,  results  in  an  oscillation,  which 
usually  is  so  small  as  to  be  harmless. 

(c)  It  may  result  from  a  defect  or  fault  in  the  circuit,  as 
an  arcing  ground  or  spark  discharge,  etc. 

One  of  the  most  serious  and  destructive  oscillations  or 
surges  is  that  produced  by  a  spark  discharge  to  ground,  or  an 
arcing  ground,  in  an  overhead  transmission  line  or  an  under- 
ground cable  system. 

Assuming  for  instance  a  44,ooo-volt  transmission  line  of 
50  miles'  length,  which  is  insulated  from  ground,  that  is,  in 
which  the  neutral  is  not  grounded.  At  44,000  volts  between 
the  line  conductors,  the  voltage  between  each  conductor  and 
the  ground,  normally,  that  is,  with  all  conductors  insulated,  is 

=  25,000.     If  now  somewhere  in  the  middle  of  this 


V    3 

line  an  insulator  breaks,  and  the  conductor  thus  drops  near 
the  grounded  insulator  pin  or  cross  arm  to  about  2  inches; 
with  25,000  volts  between  conductor  and  ground,  a  spark 
would  jump  from  the  conductor  to  the  ground,  at  the 
broken  insulator,  over  the  2 -inch  gap.  This  spark  develops 
into  an  arc,  over  which  the  electrostatic  charge  of  the  con- 
ductor discharges  to  ground  as  current,  and  the  voltage 
of  this  conductor  against  ground  thus  falls  to  zero,  since 
it  is  grounded  by  the  arc;  the  two  other  line  conductors 
then  have  the  full  line  voltage,  of  44,000,  against  ground; 
and  their  electrostatic  charge  against  ground  therefore 


84  GENERAL  LECTURES 

increases,  from  that  corresponding  to  their  normal  potential 
of  25,000,  to  that  corresponding  to  44,000  volts.  As  soon 
as  the  first  conductor  has  discharged  and  fallen  to  ground 
potential,  the  current  from  this  conductor  to  ground,  over 
the  gap,  ceases,  the  arc  goes  out,  and  the  conductor  so  is 
again  disconnected  from  ground.  It  then  begins  to  charge 
again  to  its  normal  potential  of  25,000  volts  against 
ground,  while  the  other  two  conductors  discharge,  from 
44,000  down  to  25,000  volts.  As  soon,  however,  as  during 
the  charge  the  voltage  of  the  first  conductor  has  risen  to 
the  voltage  required  to  jump  across  a  2 -inch  gap,  this 
conductor  again  discharges  to  ground  by  a  spark,  which 
develops  into  an  arc  and  so  on,  the  phenomena  of  discharge 
and  charge  of  the  conductor  repeating  continuously. 
Such  an  oscillation,  which  continues  indefinitely,  that  is, 
until  the  defect  in  the  circuit  is  remedied,  or  the  circuit 
has  broken  down  and  gone  out  of  service,  is  usually  called 
a  surge.  The  duration  of  each  oscillation  of  such  an  arcing 
ground  is  the  time  required:  (i)  To  develop  the  arc,  (2) 
to  discharge  the  line,  (3)  to  extinguish  the  arc,  (4)  to  charge 
the  line.  In  the  above  instance,  the  time  of  charge  or 
discharge  of  the  25  miles  of  line  from  the  arcing  ground  to 

25  i 

the  terminal  station  is :  — — =  -     -  second.     Assuming 

188,000       7250 

the  velocity  of  the  arc  stream  as  about  2000  feet  per  second, 

the  development  or  extinction  of  a  2  -inch  arc  would  require 

2  i 


12  X  2000       12,000 


second,     and    the   total   duration   of 


one    oscillation    therefore    is:-        —  +  -     -  +  -         -  + 

12,000       7520       12,000 

= second,  so  givin    a  frequency  of   2300  cycles. 

7520      2300 

The  two  other  lines  therefore  oscillate  in  voltage  against 
ground,  that  is,  charge  and  discharge  also  at  a  frequency  of 


HIGH  FREQUENCY  OSCILLATIONS  85 

2300  cycles.  They  receive  their  charge,  however,  over  the 
transformers  at  the  two  ends  of  the  line,  and  their  capacity 
therefore  is  in  series  with  the  self -inductance  of  these  trans- 
formers in  the  circuit  of  the  surge  frequency  of  2300  cycles; 
and  the  voltage  of  the  other  two  lines  thus  may  build  up  by 
the  combination  of  capacity  and  inductance  in  series,  to 
excessive  values;  that  is,  a  destructive  breakdown  occurs 
from  the  other  lines  to  ground — or  in  the  apparatus  con- 
nected to  them  in  the  terminal  stations  of  the  line,  as 
transformers,  current  transformers,  etc. 

A  spark  discharge  or  oscillating  ground,  therefore,  is  one 
of  the  most  serious,  as  well  as  not  infrequent  disturbances 
on  a  long-distance  transmission  line  or  underground  cable 
circuit;  and  it  is  mainly  as  a  protection  against  this  surge 
that  it  is  recommended  by  many  transmission  engineers 
to  ground  the  neutral  of  the  system  and  so  immediately 
convert  a  spark  discharge  on  one  conductor  into  a  short- 
circuit  of  one  phase  of  the  system,  and  thereby  auto- 
matically cut  out  the  circuit ;  that  is,  rather  shut  down  this 
circuit  than  continue  operation  with  an  arcing  ground  on 
the  system.  Where,  as  in  underground  cable  systems,  a 
number  of  cables  are  used  in  multiple,  the  immediate  dis- 
connection of  an  arcing  cable  undoubtedly  is  advisable. 

As  the  overhead  transmission  line  is  immersed  in  the 
electric  field  of  the  atmosphere,  any  disturbance  of  the 
atmospheric  electric  field,  whether  by  lightning  between 
cloud  and  ground,  or  between  or  within  clouds,  thus  must 
result  in  a  disturbance  and  readjustment  of  the  electrical 
condition  of  the  transmission  line.  Thus  a  discharge 
between  cloud  and  ground,  in  dropping  the  potential  dif- 
ference between  cloud  and  ground,  releases  the  bound 
electrostatic  charge  of  the  line,  and  thus  causes  a  line 
discharge  by  what  may  be  called  electrostatic  induction. 


86  GENERAL  LECTURES 

A  lightning  flash  parallel  to  the  line  electromagnetically 
induces  a  line  discharge,  etc.;  thus  potential  differences 
between  line  and  ground  occur  by  static  induction  from 
the  atmospheric  field,  resulting  in  line  discharges  and 
currents  flowing  from  the  line,  and  currents  are  induced 
electromagnetically  in  the  line  by  lightning  flashes,  etc., 
and  produce  potential  differences  and  voltages  in  the  line, 
usually  of  extremely  abrupt  character,  that  is,  of  very  high 
frequency,  where  oscillating,  reaching  hundred  thousands 
and  millions  of  cycles  per  second. 

In  general  such  very  high-frequency  disturbances,  or, 
more  correctly  speaking,  very  abrupt  disturbances,  as 
they  are  produced  by  atmospheric  lightning  as  well  as  by 
circuit  operation,  as  switching,  may  be  divided  into  three 
classes : 

(a)  Impulses,  that  is,  sudden  waves  of  voltage  or  current, 
which  are  not  oscillatory.     They   are  perhaps  the  most 
common  in  transmission  lines,  are  produced  whenever  a 
switch  is  closed  or  opened  or  any  other  change  occurs. 
While  non-oscillatory,  we  frequently  denote  their  sudden- 
ness  by    a   nominal    or   fictitious  frequency,  treating  the 
impulse  as  half  wave.     Thus  an  impulse  of  500  kilocycles 
would  be  an  impulse  in  which  the  voltage  rises  at  the  same 
rate  as  it  rises  in  a  5oo-kilocycle  oscillation  of  the  same 
maximum  voltage. 

(b)  Oscillations,    that    is,    periodic    disturbances    which 
gradually  die  out,  more  or  less  rapidly,  depending  on  the 
damping  effect  of  the  circuit  resistance. 

(c)  Cumulative  oscillations  or   surges,   that  is,   oscilla- 
tions  which    gradually   increase   in    amplitude,    until   de- 
struction of  the  circuit  occurs,  or  they  are  finally  limited 
by  increasing  energy  losses. 

The  condenser  discharge  through  an  inductive  circuit 


HIGH  FREQUENCY  OSCILLATIONS  87 

may  be  oscillatory,  or  a  single  steady  impulse  of  more  or 
less  steep  wave  front,  depending  on  the  resistance  of  the 
discharge  circuit.  With  the  transmission  line,  however, 
this  is  not  the  case,  but  in  the  same  circuit,  disturbances 
may  be  single  impulses  or  may  be  oscillations,  and  both 
usually  occur.  Which  takes  place  largely  depends  on  the 
origin  or  cause  of  the  disturbance. 

If  the  resistance  of  the  circuit  is  very  low,  that  is,  the 
damping  effect  very  small,  and  the  circuit  the  set  of  an 
induced  e.m.f.,  the  oscillation  may  become  cumulative, 
that  is,  increase  in  amplitude  and  build  up  to  a  stationary 
wave.  It  is  obvious  that  such  stationary  waves  or  cumula- 
tive oscillations  are  the  most  destructive.  Circuits,  in 
which  the  resistance  is  sufficiently  low,  compared  with  the 
inductance  and  capacity,  and  therefore  a  material  danger 
of  the  formation  of  stationary  waves  exists,  are  the  high- 
potential  windings  of  high-power  transformers,  and  in 
these,  cumulative'  oscillations  have  been  observed  and 
constitute  indeed  a  serious  source  of  danger.  Somewhat 
less  frequently,  they  also  have  been  observed  in  the  high- 
potential  armature  windings  of  large  high-voltage  alter- 
nators feeding  directly  into  overhead  lines. 


EIGHTH  LECTURE 
GENERATION 

For  driving  electric  generators  the  following  methods  are 
available : 

1.  The  hydraulic  turbine  in  a  water-power  station. 

2.  The  steam  engine. 

3.  The  steam  turbine. 

4.  The  gas  engine. 

COMPARISON  OF  PRIME  MOVERS 

i.  The  advantages  of  water  power,  compared  with  steam 
power,  are: 

(a)  Very  low  cost  of  operation;  no  fuel,  very  little 
attendance. 

The  disadvantages  are: 

(a)  Usually  the  cost  of  development  and  installation  is 
far  higher  than  with  steam  power. 

(b)  The  location  of  the  water  power  cannot  be  chosen 
freely,  but  is  fixed  by  nature ;  therefore,  the  power  cannot  be 
used  where  generated,   but  a  long-distance  transmission 
line  is  required. 

(c)  Usually  lower  reliability  of  service,  due  to  the  depend- 
ence on  a  transmission  line,  arid  on  meteorological  condi- 
tions :  the  river  may  run  dry  in  summer,  ice  interfere  with 
the  operation  in  winter. 

The  speed  of  the  water  in  the  turbine  depends  upon  the 
head  of  water,  and  is  approximately,  in  feet  per  minute, 

480 -\A>  where  h  is  the  head,  in  feet.     The  peripheral  speed 

88 


GENERATION  89 

of  the  turbine,  and  so  its  revolutions,  depends  upon  the 
speed  and  therefore  upon  the  head  of  the  water.  At  high 
heads  of  500  to  2000  feet,  as  are  found  in  the  West,  the 
electric  generators  are  thus  high-speed  machines,  of  good 
economy  and  moderate  size  and  cost.  At  low  heads, 
however,  such  as  are  usual  in  the  Eastern  States,  direct 
connection  to  a  turbine  leads  to  slow  speed  generators  of 
many  poles  and  large  size  and  cost;  while  indirect  driving, 
by  belt  or  rope,  is  mechanically  undesirable.  Very  low- 
head  water  powers  of  less  than  20  to  30  feet  head  there- 
fore usually  are  of  little  value  and  their  development  is 
economical  only  where  very  large  power  is  available,  or 
where  electric  power  is  valuable. 

Of  the  two  types  of  turbines,  the  reaction  turbine  runs 
approximately  at  the  speed  of  the  water,  and  the  action  or 
impulse  turbine  at  half  the  speed  of  the  water.  At  the 
same  head  and  thus  the  same  speed  of  the  water,  the 
reaction  turbine  gives  higher  speed,  and  is  therefore  used 
in  water  powers  of  low  and  medium  heads,  where  the  speed 
of  the  water  is  low;  while  the  impulse  turbine,  as  the 
Pelton  wheel,  is  always  used  at  very  high  heads,  at  which 
the  reaction  turbine  would  give  too  high  speeds. 

Where  water  power  is  not  available,  the  power  has  to  be 
generated  by  the  combustion  of  fuel.  In  this  case,  a  greater 
freedom  exists  in  the  choice  of  the  location  of  the  plant; 
and  it  is  located  as  near  to  the  place  of  consumption  as 
considerations  of  the  cost  of  property,  the  availability  of 
condensing  water  for  the  engines,  the  facilities  of  trans- 
portation, etc.,  permit.  Transmission  lines,  therefore, 
are  less  frequently  used,  but  in  steam  stations  of  large 
power,  high-potential  distribution  circuits  of  6600,  11,000 
or  22,000  volts,  commonly  underground  by  cables,  are 
used  in  supplying  electric  power  from  the  main  generating 


90  GENERAL  LECTURES 

station,  to  the  substations  as  centers  of  secondary  distri- 
bution (New  York,  Chicago,  etc.). 

As  source  of  power  is  available  then : 

The  steam  engine.     The  steam  turbine.     The  gas  engine. 

Comparison  of  the  steam  turbine  with  the  steam  engine: 

Some  of  the  advantages  of  the  steam  turbine  over  the 
steam  engine  are: 

(a)  High  efficiency  at  low  loads,  and  a  flatter  efficiency 
curve ;  that  is,  the  turbine  efficiency  remains  high  at  partial 
loads,  and  at  overloads,  where  the  steam  engine  efficiency 
falls  off  greatly ;  so  that  the  superiority  of  the  steam  turbine 
in  efficiency,  while  great  at  rated  load,  is  still  far  greater  at 
partial  load,  light  load  and  overload. 

(b)  Smaller  size,  weight  and  space  occupied. 

(c)  Uniform  rate  of  rotation,  therefore,  decreased  liability 
of  hunting  of  synchronous  machines,  and  decreased  neces- 
sity   of    heavy    foundations    to    withstand    reciprocating 
strains. 

(d)  Greater  reliability  of  operation  and  far  less  attend- 
ance required. 

The  steam  turbine  reaps  a  far  greater  benefit  in  economy 
than  the  steam  engine  from  superheat  of  the  steam,  and 
from  a  high  vacuum  in  the  condenser. 

The  disadvantage  of  the  steam  turbine  is  that  in  smaller 
units,  the  superiority  of  the  steam  turbine  over  the  steam 
engine  in  efficiency,  that  is,  in  steam  consumption,  is  less 
marked,  and  when  operating  non-condensing,  the  simple 
turbine  offers  little  advantage  in  economy  in  small  units. 
Therefore,  in  units  up  to  a  few  hundred  horsepower,  the 
reciprocating  steam  engine  still  finds  a  large  field  in  driving 
electric  generators,  in  isolated  stations,  etc.,  while  for  larger 
units  of  thousands  of  kilowatt  output,  the  reciprocating  steam 
engine  has  entirely  been  superseded  by  the  steam  turbine. 


GENERATION  91 

The  speed  characteristic  of  the  steam  turbine  is  similar 
to  that  of  the  constant-voltage  direct-current  shunt  motor, 
or  the  polyphase  induction  motor ;  while  that  of  the  recipro- 
cating steam  engine  is  similar  to  that  of  the  series  motor. 
That  is,  to  produce  the  same  torque,  the  steam  turbine 
requires  approximately  the  same  amount  of  steam,  irre- 
spective of  the  speed;  therefore,  its  efficiency  is  highest 
at  a  certain  speed,  or  rather  range  of  speed,  but  falls  off  with 
the  speed ;  while  the  steam  consumption  of  the  reciprocating 
engine,  at  constant  torque,  is  approximately  proportional 
to  the  speed,  that  is  the  number  of  times  the  cylinders  are 
filled  per  minute.  Or  in  other  words,  the  torque  per  pound 
of  steam  used  per  minute  is  approximately  constant  and 
independent  of  the  speed  in  the  turbine  (just  as  the  torque 
per  volt-ampere  is  approximately  constant  for  all  speeds 
in  the  induction  motor),  while  in  the  reciprocating  en- 
gine the  torque  per  pound  of  steam  used  per  minute  is 
approximately  inversely  proportional  to  the  speed,  or  at 
least  greatly  increases  with  decrease  of  speed  (just  as  in 
the  series  motor  the  torque  per  volt-ampere  input  increases 
with  decrease  of  speed). 

The  steam  turbine,  therefore,  would  not  be  suitable  for 
directly  driving  a  railway  train  in  rapid  transit  service, 
but  is  suitable  for  driving  the  ship's  propeller. 

Just  as  in  the  induction  motor  a  series  of  economical 
speeds  can  be  produced  by  changing  the  number  of  poles, 
so  in  the  steam  turbine  a  series  of  economical  speeds  can  be 
produced  by  changing  the  number  of  expansions.  For 
driving  electrical  machinery  this,  however,  is  of  no 
importance. 

Comparison  of  the  gas  engine  with  the  steam  turbine  and 
the  steam  engine. 

The  leading  and  foremost  advantage  of  the  gas  engine, 


92  GENERAL  LECTURES 

and  the  feature  which  gives  it  the  right  of  existence,  is  its 
high  efficiency.  That  is,  the  same  amount  of  coal,  con- 
verted to  gas  and  fed  to  a  good  gas  engine,  gives  more 
power  than  when  burned  under  the  boilers  of  the  engine, 
except  perhaps  the  very  large  and  exceedingly  efficient 
steam-turbine  units.  The  cause  is  that  the  gas  engine 
works  over  a  far  greater  temperature  range  than  the  steam 
engine  and  even  the  steam  turbine — although  the  latter, 
by  its  ability  to  economically  utilize  superheat  and  high 
condenser  vacuum,  gets  the  benefit  of  a  larger  temperature 
range  over  the  steam  engine. 

If,  therefore,  the  gas  engine  were  not  so  very  greatly 
handicapped  in  every  other  respect,  it  would  long  have 
superseded  the  steam  engine  and  even  the  steam  turbine, 
except  perhaps  in  the  largest  sizes. 

The  disadvantages  of  the  gas  engine  in  every  respect  but 
efficiency  are  such,  however,  that  in  spite  of  its  existence  of 
over  half  a  century  it  has  not  made  a  serious  impression  on 
the  industry ;  while  the  steam  turbine  during  the  short  time 
of  its  existence  has  entirely  replaced  the  steam  engine  in 
large  electric  generating  plants. 

The  cause  of  the  disadvantages  of  the  gas  engine  is  the 
high  maximum  temperature  and  the  high  maximum 
pressure  compared  with  the  mean  pressure  in  the  cylinders, 
which  is  necessary  to  get  the  greater  temperature  range  and 
thus  the  efficiency,  therefore,  is  inherent  in  this  type  of 
apparatus. 

The  output  depends  upon  the  mean  pressure  in  the  cylin- 
der, which  is  relatively  low;  the  strains  on  the  maximum 
pressure,  which  is  -very  high;  and  the  gas  engine,  therefore, 
must  be  very  large,  and  its  moving  parts  very  strong  and 
heavy,  for  its  output.  The  impulse  due  to  the  rapid  pres- 
sure change  is  very  jerky — almost  of  the  nature  of  an  ex- 


GENERATION  93 

plosion — and  the  steadiness  of  the  rate  of  rotation  is,  there- 
fore, very  low,  requiring  for  electric  driving  very  heavy  fly- 
wheels and  numerous  cylinders. 

Compared  with  the  steam  engine,  the  disadvantages  of 
the  gas  engine  so  are: 

(a)  Lower   reliability;   higher    cost    of   maintenance   in 
attendance,  repairs,  and  greater  depreciation. 

(b)  Larger  size  and  space  occupation  for  the  same  output. 

(c)  Less  easy  to  start. 

(d)  In  general,  lower  steadiness  of  the  rate  of  rotation. 
The  advantage  of  the  gas  engine  is,  that  it  requires  no 

boiler  plant;  the  compensating  disadvantage,  that  it  re- 
quires a  gas-generating  plant.  This  latter  disadvantage 
disappears  where  gas  is  available  as  fuel — in  the  waste 
gases  of  blast  furnaces  of  steel  plants  and  in  the  natural 
gas  districts — and  in  those  cases  gas  engines  have  found 
their  introduction.  They  have  also  been  installed  for 
smaller  powers,  where  low  cost  of  fuel  is  unessential,  but 
the  operation  of  a  steam  boiler  is  objectionable,  as  in  iso- 
lated plants  using  city  gas  or  liquid  fuel  (gasolene,  etc.). 

In  general,  however,  with  the  exception  of  those  special 
cases,  the  gas  engine  does  not  yet  come  into  consideration 
in  the  electric-power  generating  station. 

ELECTRIC  GENERATORS 

In  general,  considerations  of  economy  make  it  desirable 
to  generate  the  electric  power  in  the  form  in  which  it  is 
used.  In  most  cases,  however,  this  is  not  feasible,  but  a 
higher  voltage  or  even  a  different  form  of  power  (alternating 
instead  of  direct)  is  necessary  in  the  generating  station 
than  that  required  by  the  user,  to  enable  transmission  and 
distribution;  and  then  usually  three-phase  alternating 
current  is  generated. 


94 


GENERAL  LECTURES 


i.  For  isolated  plants,  and  in  general  distribution  of 
such  small  extent  as  to  be  within  range  of  220-volt  distribu- 
tion, 22o-volt  direct-current  generators  are  used,  operating 
a  three- wire  system,  formerly  two  no-volt  machines,  sup- 
plying the  two  sides  of  the  system,  now  almost  always  200- 
volt  machines,  deriving  the  neutral  by  equalizer  machines, 
or  by  connection  to  a  storage  battery,  or  by  compensator 
and  collector  rings  on  the  22o-volt  generator.  That  is, 
two  diametrically  opposite  (electrically)  points  of  the 
armature  winding  are  connected  to  collector  rings  (so  giving 


FIG.  24. — Three-wire  generator. 

an  alternating-current  voltage  on  those  collector  rings),  an 
alternating-current  auto-transformer  (transformer  with  a 
single  winding)  is  connected  between  the  collector  rings, 
and  the  neutral  brought  out  from  the  center  of  the  auto- 
transformer,  as  shown  diagrammatically  in  Fig.  24.  This 
arrangement  is  now  most  commonly  used. 

For  direct-current  distribution  in  larger  cities,  such 
generating  stations  have  practically  disappeared,  and  have 
been  replaced  by  converter  substations,  receiving  power 
from  a  main  generating  station,  as  three-phase  alternating 


GENERATION  95 

current  of  6600,  11,000  or  20,000  volts,  and  25  or  60 
cycles.  They  are  used,  however,  very  largely  for  isolated 
plants,  in  large  office  buildings,  apartment  houses,  etc. 

2.  For  street  railway,  6oo-volt  direct-current  generators 
are  still  used  to  some  extent,  where  the  railway  system  is 
of  moderate  extent.     In  large  railway  systems,  and  roads 
covering    greater    distances,    as    interurban    trolley    lines, 
direct  generation  of  600  volts  direct  current  has  practically 
disappeared  before  the  railway  converter  substation,  re- 
ceiving   power    as    three-phase    alternating    from    trans- 
mission lines  or  liigh-voltage  distribution  cables. 

3.  For  general  distribution  by  alternating  current,  with 
a  22oo-volt  primary  system,  direct  generation  is  still  to  a 
considerable   extent   used,    often  by   a  four-wire  primary 
system  (page  32,  §4),  as  the  use  of  2 200- volt  permits  the 
system  to  cover  a  very  large  territory,  and  substations  are 
mainly  used  only  where  the  power  can  be  derived  from  a 
long-distance   transmission   line,    or   where   the    22oo-volt 
distribution  is  only  a  part  of  a  large  system  of  electric 
generation;  as  in  the  suburban  distribution  of  large  cities, 
using  converter  substations  for  the  interior.     In  this  case, 
where  the  transmission  line  or  the  main  generating  station 
is  at  60  cycles,  large  station  transformers  are  used  for  the 
supply   of   the    22oo-volt   distribution;   where   the   power 
supply  is   at   25    cycles,    either  frequency   converters,    or 
motor  generators  change  to  60  cycles,  2200  volts. 

4.  For  special  use,  as  for  electrochemical  work,  where  the 
electric  power  is  generated  directly,  different  voltages,  etc., 
may  be  used  to  suit  the  requirements. 

Where  the  power  cannot  be  generated  in  the  form  in 
which  it  is  used,  and  that  is  the  case  in  all  larger  systems, 
three-phase  alternators  are  almost  universally  used. 

The    single-phase    system    has    the    disadvantage    that 


96  GENERAL  LECTURES 

single-phase  induction  and  synchronous  motors  and  con- 
verters are  inferior  to  polyphase  machines,  and  single- 
phase  alternators  larger  and  less  efficient,  and  for  lighting, 
where  single-phase  is  preferable,  single-phase  lighting 
circuits  can  be  operated  from  polyphase  alternators. 

It  must  be  considered  that  in  the  modern  large  generating 
system,  the  lighting  load  often  is  only  a  small  part  of  the 
total  load. 

Two-phase  also  has  practically  gone  out  of  use,  since  it 
offers  no  advantage  over  the  three-phase,  and  the  three- 
phase  is  preferable  for  transmission,  requiring  only  three 
conductors,  while  two-phase  requires  four. 

In  polyphase  alternators  the  flow  of  power  is  constant, 
that  is,  at  any  moment  adding  the  power  of  all  phases  gives 
the  same  value,  while  in  single-phase  alternators  the  power 
is  pulsating. 

In  a  polyphase  machine  the  armature  reaction  also  is  con- 
stant, in  a  single-phase  machine,  pulsating;  in  the  latter 
therefore,  in  machines  of  very  large  armature  reaction,  as 
turbo-alternators,  pulsations  of  the  magnet  field,  and 
thereby  loss  in  efficiency,  and  heating  may  result. 

An  alternator  has  armature  reaction  and  self-induction. 

The  armature  reaction  is  the  magnetic  action  of  the  arma- 
ture current  on  the  field,  that  is,  the  armature  current 
demagnetizes  or  magnetizes  the  field  according  to  its  phase, 
and  so  lowers  or  raises  the  voltage.  Armature  reaction, 
therefore,  is  expressed  in  ampere- turns. 

Self-induction  is  the  action  of  the  armature  current  in 
producing  magnetism  in  the  armature,  which  magnetism 
does  not  go  through  the  field.  This  magnetism  induces  an 
e.m.f.  in  the  armature,  which  opposes  or  assists  the  e.m.f. 
produced  by  the  field  magnetism,  according  to  the  phase 
of  the  armature  current,  and  so  lowers  or  raises  the  volt  age. 


GENERATION  97 

Self-induction,  or  "armature  reactance"  therefore  is  ex- 
pressed in  ohms. 

Armature  reaction  and  self-induction  therefore  act  in  the 
same  manner,  lowering  the  voltage  with  lagging  and  raising 
the  voltage  with  leading  current. 

In  calculating  alternators,  either  the  armature  reaction 
and  the  self-induction  can  both  be  considered,  which  makes 
the  calculation  more  complicated ;  or  the  armature  reaction 
may  be  neglected  and  the  self-induction  made  so  much 
larger  as  to  allow  for  the  armature  reaction.  This  self- 
induction  is  then  called  the  "synchronous  reactance" 
and,  combined  with  the  armature  resistance,  the  "synchro- 
nous impedance"  of  the  machine.  Or  the  self-induction 
may  be  neglected  and  only  the  armature  reaction  considered, 
but  which  is  increased  to  allow  for  the  self-induction. 

The  last  way  (armature  reaction),  is  used  in  designing 
machines;  the  second  way  (synchronous  reactance)  in  cal- 
culations with  machines  and  systems. 

In  the  momentary  short-circuit  current  of  alternators, 
however,  the  armature  reaction  and  the  self-induction  must 
be  considered  separately,  since  they  act  differently. 

In  the  moment  of  short-circuiting  an  alternator,  the  self- 
induction  acts  immediately  in  limiting  the  current,  but  not 
so  the  armature  reaction,  because  it  takes  time  before  the 
armature  current  demagnetizes  the  field,  that  is,  the  field 
exciting  winding  acts  as  a  short-circuited  secondary  around 
the  field  poles,  and  retards  the  decrease  of  field  magnetism 
resulting  from  the  demagnetizing  action  of  the  armature 
current  by  inducing  a  current  in  the  field  winding,  which 
tends  to  maintain  the  field  magnetism. 

Therefore,  in  the  first  moment  after  the  short-circuit 
the  armature  current  is  limited  by  self-induction  only,  and 


98  GENERAL  LECTURES 

is  therefore  much  larger  than  afterwards,  when  self-induc- 
tion and  armature  reaction  both  act. 

In  machines  of  low  armature  reaction  and  high  self- 
induction,  as  high-frequency  alternators,  the  momentary 
short-circuit  current  is  not  much  larger  than  the  permanent 
short-circuit  current.  In  machines  of  low  self-induction, 
that  is,  of  a  well-distributed  armature  winding,  but  high 
armature  reaction,  (that  is,  very  large  output  per  pole,  as  in 
steam  turbine  alternators),  the  momentary  short-circuit 
current  may  be  many  times  greater  than  the  permanent 
value  of  the  short-circuit  current,  which  is  reached  after 
a  few  seconds. 

In  the  moment  of  short-circuiting  such  an  alternator,  the 
field  current  rises  to  several  times  its  normal  value,  and 
becomes  pulsating.  Gradually  the  armature  current  and 
the  field  current  die  down  to  their  normal  values. 

Since  the  regulation  of  such  alternators  mainly  depends 
upon  the  armature  reaction,  which  is  very  large  compared 
with  the  self-induction,  even  a  considerable  external  self- 
induction  inserted  as  reactive  coil  for  limiting  the  momen- 
tary short-circuit  current  does  not  much  increase  the  com- 
bined effect  of  armature  reaction  and  self-induction;  that 
is,  does  not  seriously  affect  the  regulation,  and  besides,  in 
very  large  systems,  the  regulation  of  the  generators  is 
immaterial. 

In  large  steam-turbine  alternators,  the  momentary  short- 
circuit  current  may  reach  20  to  30  times  full-load  current, 
and  in  large  generating  stations,  having  a  number  of  such 
turbo-alternators  feeding  into  the  busbars,  a  short-circuit 
at  or  near  the  busbars  thus  may  cause  momentarily  cur- 
rents to  flow,  amounting  to  several  million  kilo  volt- amperes, 
and  therefore  extremely  destructive  by  their  energy,  in 
circuit-breakers,  etc.,  and  destructive  by  the  mechanical 


GENERATION  •  99 

magnetic  forces  in  machines,  transformers,  etc.  It  there- 
fore becomes  necessary  and  has  become  the  practice  in 
such  large  high-power  systems,  to  limit  the  momentary 
short-circuit  currents  by  inserting  power  limiting  react- 
ances into  the  generator  leads,  and  in  very  large  systems 
also  in  the  feeder  circuits,  and  to  divide  the  busbars  into 
sections  by  busbar  reactances.  Such  power-limiting  re- 
actances must  be  built  so  as  not  to  saturate  magnetically 
even  at  short-circuit  current,  and  therefore  are  built  with- 
out iron,  that  is,  with  air  cores. 


NINTH  LECTURE 
HUNTING  OF  SYNCHRONOUS  MACHINES 

Cross-currents  can  flow  between  alternators  due  to  dif- 
ferences in  voltage,  that  is,  differences  in  excitation;  and 
due  to  differences  in  phase,  that  is,  differences  in  position 
of  their  rotors. 

Cross-currents  due  to  differences  in  excitation  are  watt- 
less currents,  magnetizing  the  underexcited  and  demag- 
netizing the  overexcited  machine. 

Cross-currents  due  to  differences  in  position  are  energy 
currents,  accelerating  the  lagging  and  retarding  the  leading 
machine.  Their  magnetic  action  is  a  distortion  or  a  shift 
of  the  field,  that  is,  they  increase  the  magnetic  density  at 
the  one  and  decrease  it  at  the  other  pole  corner. 

If  two  machines  are  thrown  together  out  of  phase,  or 
brought  out  of  the  phase  by  some  cause  (as  the  beat  of  an 
engine,  or  the  change  of  load  of  a  synchronous  motor)  then 
the  two  machines  pull  each  other  in  phase  again,  oscillate 
a  few  times  against  each  other,  which  oscillation  gradually 
decreases  and  dies  out,  and  the  machines  run  steadily. 

If  the  oscillations  do  not  decrease,  but  continue,  the 
machines  are  said  to  be  hunting. 

If  the  oscillation  is  small  it  may  do  no  harm;  if  it  is 
greater,  it  may  cause  fluctuation  of  voltage,  resulting  in 
flickering  of  lights,  etc. ;  if  it  gets  very  large,  it  may  throw 
the  machines  out  of  step. 

Some  causes  of  hunting  are: 

First. — Magnetic  lag. 

100 


HUNTING  OF  SYNCHRONOUS  'MACMNES:  :  101 

Second. — Pulsation  of  engine  speed. 

Third. — Hunting  of  engine  governors. 

Fourth. — Wrong  speed  characteristic  of  engine. 

First. — When  the  machines  move  apart  from  each  other, 
magnetic  attraction  opposes  their  separation.  When  they 
pull  together  again,  magnetic  attraction  pushes  them  "to- 
gether with  the  same  force,  so  that  they  would  move  over 
the  position  of  coincidence  in  phase  and  separate  again  in 
the  opposite  direction  just  as  much  as  before. 

Energy  losses  as  friction,  etc.,  retard  the  separation  and 
so  make  them  separate  less  than  before,  every  time  they  do 
so,  that  is,  cause  them  gradually  to  stop  seesawing. 

If,  however,  there  is  a  lag  in  the  magnetic  attraction,  then 
they  come  together  with  greater  force  than  they  separated, 
so  separate  more  in  the  opposite  direction,  that  is,  the 
oscillation  increases  until  the  machines  fall  out  of  step, 
or  the  further  increase  of  oscillation  is  stopped  by  the 
increasing  energy  losses. 

This  kind  of  hunting  is  stopped  by  increasing  the  energy 
losses  due  to  the  oscillation,  by  copper  bridges  between  the 
poles,  by  aluminum  collars  around  the  pole  faces,  or  most 
effectively  by  a  complete  squirrel -cage  winding  in  the  pole 
faces. 

The  frequency  of  this  hunting  depends  on  the  magnetic 
attraction,  that  is,  on  the  field  excitation,  and  on  the  weight 
of  the  rotating  mass.  The  higher  the  field  excitation  the 
greater  is  the  magnetic  force,  that  is,  quicker  the  motion 
of  the  machine  and  therefore  the  higher  the  frequency. 
The  greater  the  weight,  the  slower  it  is  set  in  motion,  that 
is,  the  lower  the  frequency. 

Characteristic  of  this  hunting  therefore  is  that  its  fre- 
quency is  changed  by  changing  the  field  excitation. 

Second. — If  the  speed  of  the  engine  varies  during  the  rota- 


102  '          £RAL  LECTURES 

tion,  rising  and  falling  with  the  steam  impulses,  then  the 
alternator  speed  and  the  frequency  also  pulsate  with  a 
speed  equal  to,  or  a  multiple  of  the  engine  speed.  If  now 
two  such  alternators  happen  to  be  thrown  together  so  that 
the  moment  of  maximum  frequency  of  one  coincides  with 
the  moment  of  minimum  frequency  of  the  other,  the  two 
machines  cannot  run  in  perfect  phase  with  each  other, 
but  pulsate,  alternatingly  getting  out  of  phase  with  each 
other,  coming  together,  and  getting  out  again  in  the  oppo- 
site direction.  If  the  deviation  of  the  two  engines  from 
uniform  rate  of  rotation  is  very  little — the  maximum  dis- 
placement of  the  alternator  from  the  position  of  uniform 
rotation  not  more  than  three  electrical  degrees — the  pul- 
sating cross-currents,  which  flow  between  the  alternators, 
are  moderate,  and  the  phenomenon  harmless,  as  long 
as  the  oscillation  is  not  cumulative.  An  increase  of  the 
weight  of  the  flywheel  of  the  engine  decreases  the  speed  pul- 
sation and  thereby  decreases  this  form  of  hunting,  which  is 
the  most  harmless,  but  increases  the  tendency  to  the  hunt- 
ing in  No.  i  and  No.  3,  and  therefore  is  not  desirable;  but 
steadiness  of  engine  speed  should  be  secured  by  the  design 
of  the  engine,  that  is,  by  balancing  the  different  forces  in 
the  engine,  as  the  steam  impulses  and  the  momentum  of  the 
reciprocating  masses,  so  as  to  give  a  uniform  resultant. 

In  such  a  case,  when  running  from  a  single  alternator, 
driven  by  a  reciprocating  engine  with  moderate  speed  pulsa- 
tion (therefore  receiving  a  slightly  pulsating  frequency),  a 
synchronous  motor  without  anti-hunting  devices,  but  of 
high  armature  reaction,  and  therefore  high  stability,  may 
run  very  steadily,  with  no  appreciable  current  pulsation; 
while  the  same  synchronous  motor,  when  supplied  with 
a  squirrel-cage  winding  in  the  field  pole  faces  as  the  most 
powerful  anti-hunting  device,  may  show  pulsation  in  the 


HUNTING  OF  SYNCHRONOUS  MACHINES       103 

current  supplied,  which  in  a  high-speed  motor,  of  high 
momentum,  may  be  considerable.  The  cause  is,  that 
in  the  former  case  the  synchronous  motor  does  not  follow 
the  pulsation  of  frequency,  but  keeps  constant  speed,  while 
in  the  latter  case  the  squirrel-cage  winding  forces  the  motor 
to  follow  the  variation  in  frequency  by  accelerating  and 
decelerating,  and  the  pulsation  of  the  current  therefore 
is  not  hunting,  but  energy  current  required  to  make  the 
motor  speed  follow  the  engine  pulsation. 

If  the  frequency  of  oscillation  of  the  machine  (as  deter- 
mined by  its  field  excitation  and  the  weight  of  its  moving 
part)  is  the  same  as  the  frequency  of  engine  impulses,  that 
is,  the  same  as  the  number  of  engine  revolutions  or  a  multi- 
ple thereof,  then  successive  engine  impulses  will  always 
come  at  the  same  moment  of  the  machine  beat  and  so  con- 
tinuously increase  it:  that  is,  the  machine  oscillation  in- 
creases, or  the  machine  hunts. 

In  this  case  of  cumulative  hunting  caused  by  the  engine 
impulses,  the  frequency  of  oscillation  agrees  with  the 
engine  oscillation. 

Third. — If  one  alternator  is  a  little  ahead,  that  is,  takes 
a  little  more  load,  its  engine  governor  regulates  by  reducing 
the  steam,  slowing  down  the  alternator  to  its  normal  posi- 
tion. When  slowing  down,  the  flywheel  is  giving  power, 
therefore  the  steam  supply  has  been  reduced  more  than 
it  should  be,  that  is,  the  alternator  drops  behind  and  takes 
less  load  until  the  governor  has  admitted  steam  again. 

In  the  meantime,  while  the  first  alternator  was  behind 
and  took  less  load,  the  second  alternator  had  to  take  the 
load,  that  is,  the  governor  of  the  second  alternator  ad- 
mitted more  steam.  When  the  first  alternator  has  picked 
up  again  to  its  normal  load,  the  second  alternator  gets  too 
much  steam  and  its  governor  must  cut  off,  but  then  cuts 


104  GENERAL  LECTURES 

off  too  much,  the  same  way  as  the  first  alternator  did  be- 
fore; so  the  two  governors  hunt  against  each  other  by 
alternatingly  admitting  too  much  and  too  little  steam. 

In  this  case  the  frequency  of  hunting  does  not  depend  on 
the  engine  speed  and  does  not  vary  much  with  the  field 
excitation,  but  the  hunting  is  usually  much  less  at  heavy 
load  than  at  light  load.  The  reason  is  that  at  load,  when 
the  engines  take  much  steam,  a  little  change  in  the  steam 
supply  does  not  make  so  much  difference  as  at  light  load, 
where  the  engines  take  very  little  steam,  and  so  a  small 
change  of  the  governor  has  a  great  effect. 

Fourth. — To  run  in  parallel,  the  speed  of  the  engines 
driving  the  alternators  must  decrease  with  the  load  so  that 
the  alternators  divide  the  load. 

If  the  speed  did  not  change  with  the  load,  then  there 
would  be  no  division  of  the  load ;  the  one  engine  could  take 
all  the  load,  the  other  nothing. 

If  the  speed  curve  of  the  engine  is  such  that  the  speed 
does  not  fall  off  much  between  no  load  and  moderate  load, 
then  the  alternators  will  not  well  divide  the  load  at  light 
loads,  and  hunt  while  running  in  parallel  at  light  load,  but 
steady  down  at  heavier  loads. 

To  distinguish  between  different  kinds  of  hunting: 

First. — Change  of  frequency  with  change  of  field  excita- 
tion points  to  magnetic  hunting,  especially  if  very  marked. 

Second. — Equality  of  frequency  with  the  generator  speed 
points  to  engine  hunting. 

Third. — If  the  synchronous  motor  or  converter  steadies 
down  when  only  one  engine  is  running,  it  points  to  engine 
governor  hunting. 

Fourth. — Steadiness  of  operation  at  load,  and  unsteadi- 
ness at  light  load  points  to  governor  hunting,  but  may 
also  be  due  to  engine  and  magnetic  hunting. 


HUNTING  OF  SYNCHRONOUS  MACHINES         105 

Fifth. — If  by  disconnecting  one  governor  and  governing 
one  engine  only,  the  hunting  disappears,  then  it  is  due  to 
governor  hunting.  If  it  does  not  disappear,  then  both 
governors  may  be  disconnected  and  the  engines  run  care- 
fully without  governors,  by  throttle.  If  the  hunting  then 
disappears,  it  is  due  to  the  governors ;  if  it  does  not  disap- 
pear, it  is  probably  magnetic  hunting. 

If  by  making  the  field  excitation  of  the  two  alternators 
or  two  converters  that  hunt,  unequal — by  increasing  the 
one  and  decreasing  the  other — the  hunting  disappears  or 
decreases,  it  is  magnetic  hunting. 

In  a  case  of  hunting,  the  following  points  should  be 
investigated : 

A.  HUNTING  OF  SYNCHRONOUS  MOTORS  OR  CONVERTERS 

First. — Count  the  number  of  beats  to  get  the  frequency 
of  hunting.  If  the  beats  periodically  increase  and  decrease, 
it  shows  two  frequencies  of  hunting  superimposed  upon 
each  other.  Then  count  the  total  number  of  beats  per 
minute  (counting  during  intermissions)  and  count  the  num- 
ber of  intermissions  per  minute. 

The  two  frequencies  are  the  number  of  beats  per  minute, 
plus  and  minus  half  the  number  of  intermissions  or  nodes 
per  minute. 

Instance:  80  beats  per  minute,  10  intermissions  per 
minute.  Frequencies  80  +  5  and  80  —  5  or  85  and  75 
beats. 

If  one  of  the  two  frequencies  approximately  coincides 
with  the  engine  speed,  it  can  be  assumed  as  the  engine 
speed.  The  number  of  revolutions  of  the  engine  obviously 
should  be  counted  also. 

Second. — See  whether  any  machine  in  the  system  runs 
at  a  speed  equal  to  the  observed  frequency  of  hunting. 


106  GENERAL  LECTURES 

For  instance,  a  generator  may  make  75  revolutions  per 
minute,  which  accounts  for  this  frequency. 

Third. — With  several  converters  in  the  same  station  see 
whether  the  station  ammeter  also  hunts. 

If  the  station  ammeter  is  very  steady  and  the  converter 
ammeters  hunt,  the  converters  hunt  against  each  other. 
In  this  case  lowering  the  one  and  raising  the  other  con- 
verter field  and,  if  necessary,  readjusting  the  potential  regu- 
lators, may  stop  the  hunting  by  giving  the  two  machines 
different  frequencies  of  hunting  which  interfere  with  each 
other. 

If  all  three  meters  are  unsteady,  the  converters  may  hunt 
against  each  other  or  hunt  together  against  another  station 
or  against  the  generator.  Then  find  out  whether  the 
ammeter  needles  of  both  converters  go  up  and  down  to- 
gether or  one  goes  up  when  the  other  goes  down. 

Fourth. — Change  the  field  excitation  and  see  whether  the 
change  of  field  excitation  changes  the  frequency.  See 
whether  a  decrease  of  field  excitation  steadies  it.  Occa- 
sionally hunting  can  be  stopped  by  lowering  the  field 
excitation,  that  is,  running  with  lagging  current. 

Fifth. — If  several  converters  of  a  substation  feed  into  the 
same  direct-current  system,  as  the  converters  of  other  sub- 
stations, disconnect  the  direct-current  sides  of  the  converters 
and  see  if  they  still  hunt. 

If  two  or  more  converters  run  in  the  same  station,  run 
only  one  and  see  whether  it  hunts. 

Cure. — First. — If  the  hunting  is  magnetic  hunting  be- 
tween converters  or  synchronous  motors,  it  is  frequently 
reduced  by  making  the  field  excitation  unequal,  or  put- 
ting a  flywheel  on  one  converter,  or  belting  some  other 
machine  to  it,  or  running  an  induction  motor  in  the  same 
station  or  in  any  other  way  breaking  up  the  resonance. 


HUNTING  OF  SYNCHRONOUS  MACHINES       107 

Second. — Several  converters  hunting  against  each  other 
in  the  same  substation  are  frequently  steadied  by  con- 
necting the  collector  rings  with  each  other,  that  is,  by 
equalizer  connections  between  converter  and  transformer 
or  regulator. 

In  this  case  the  commutator  brushes  have  to  be  carefully 
adjusted  to  avoid  sparking. 

Third. — The  most  effective  way  is  to  put  copper  bridges 
on  the  converters  or  synchronous  motors,  or  better  still  a 
squirrel-cage  winding  in  the  field  pole  faces. 

Not  so  good  are  short-circuiting  rings  around  the  field 

poles. 

B.  HUNTING  OF  GENERATORS 

First. — Count  the  frequency  in  the  same  way  as  before. 

Second. — See  whether  the  frequency  agrees  with  the 
generator  speed  or  with  the  speed  of  some  large  motor  on 
the  system. 

Third. — See  whether  the  frequency  changes  with  the 
excitation. 

Fourth. — See  whether  the  hunting  changes  with  the  load, 
that  is,  gets  worse  at  light  load. 

Fifth. — Disconnect  governors  and  see  whether  this  stops 
hunting. 

Cure. — First. — If  the  hunting  stops  when  disconnecting 
the  governors,  it  is  hunting  of  the  governors  and  can  be 
cured  by  putting  a  stiff  dashpot  on  the  governors. 

Second. — If  the  hunting  does  not  stop  by  disconnecting 
the  governors,  copper  bridges  on  the  alternators  will 
cure  it. 

Third. — If  the  hunting  has  the  speed  of  the  engine,  it  may 
be  reduced  by  increasing  the  flywheel  or  decreasing  it,  by 
running  an  induction  motor  in  the  station,  or  in  any  other 
way  breaking  up  the  resonance. 


108  GENERAL  LECTURES 

In  general,  systems  having  all  kinds  of  loads,  different 
sizes  of  generators,  motors  and  converters,  induction 
motors  and  synchronous  motors  mixed,  etc.,  are  very  little 
liable  to  hunting.  Hunting  is  most  liable  to  occur  when 
all  the  generators  are  of  the  same  kind  and  all  the  syn- 
chronous motors  or  converters  are  of  the  same  kind. 

Resistance  in  general  increases  the  tendency  to  hunting 
so  that  if  the  resistance  drop  is  more  than  10  to  15 
per  cent.,  special  precautions  have  to  be  taken,  such  as 
squirrel-cage  pole-face  windings,  or  synchronous  machines 
must  be  altogether  avoided  and  induction  motor-generator 
sets  used. 

Reactance  between  the  machines  decreases  the  tendency 
except  when  very  large. 

The  tendency  to  hunting  is  more  severe  at  the  end  of 
long-distance  transmission  lines  and  induction  machines 
are  therefore  often  preferable  in  such  a  place  or  synchronous 
machines  with  squirrel-cage  pole-face  winding. 

Machines  with  high  armature  reaction  are  much  less 
liable  to  hunt  than  machines  with  low  armature  reaction, 
that  is,  close  regulation,  because  with  high  armature  reac- 
tion the  current  varies  much  less  with  a  change  of  position 
of  the  machine.  Therefore,  6o-cycle  converters  are  more 
liable  to  hunt  than  25-cycle  converters,  because  in  60- 
cycle  converters  there  is  not  so  much  space  on  the  armature 
to  get  high  armature  reaction. 


TENTH  LECTURE 
REGULATION  AND  CONTROL 

A.  DIRECT-CURRENT  SYSTEMS 

In  direct-current  three-wire  22o-volt  distribution  systems 
several  outside  busbars  are  used  and,  with  change  of  load, 
the  feeders  are  changed  from  one  busbar  to  another. 

The  different  busbars  are  connected  to  different  ma- 
chines, to  the  storage  battery  or  to  boosters. 

The  lighting  boosters  are  low- volt  age  machines  sepa- 
rately excited  from  the  busbars.  The  main  generators  are 
shunt  machines  or  rather  are  excited  from  the  busbars,  or 
rotary  converters,  and  are  usually  of  250  volts,  that  is,  the 
neutral  brought  out  by  collector  rings  and  compensator. 

In  railway  circuits,  in  addition  to  trolley  wire  and  rail 
return,  trolley  feeders  and  ground  feeders,  or  plus  and 
minus  feeders  are  sufficient  for  converter  substations,  and 
where  the  distance  gets  too  great  for  feeders,  another  sub- 
station is  installed. 

When  using  direct-current  generators,  series  boosters  are 
used  to  feed  very  long  feeders  which  otherwise  would  have 
an  excessive  drop  of  voltage.  In  this  way  feeder  drops  of 
200  to  300  volts  are  taken  care  of  by  the  railway  booster. 
Such  a  large  voltage  drop  is  uneconomical  and  railway 
boosters  are  therefore  used  only  for  small  sections  for  which 
it  does  not  pay  to  install  a  separate  station,  especially  where 
the  load  is  very  temporary,  as  for  instance,  heavy  Sunday 
load,  etc. 

Railway  boosters  are  series  machines,  that  is,  the  series 

field  and  the  machine  voltage  therefore  are  proportional 

109 


110  GENERAL  LECTURES 

to  the  current.  In  such  railway  boosters  it  is  necessary 
to  take  care  in  the  booster  design  that  it  does  not  build  up 
as  series  generator  feeding  a  current  through  the  local 
circuit  between  a  short  feeder  and  a  long  feeder,  as  shown 
in  Fig.  25. 

A  series  machine  excites  if  the  resistance  of  its  circuit  is 
less  than  a  certain  critical  value.  To  avoid  such  local 
circuit,  either  the  trolley  circuit  is  cut  between  the  feeders, 
or  the  boosting  kept  below  the  critical  value. 


sews  ffoo-sre* 


FIG.   25. — Series  railway  booster. 

This  feature  of  building  up  and  short-circuiting,  if  the 
resistance  of  the  circuit  is  too  low,  is  characteristic  of  all 
series  boosters,  and  to  be  looked  out  for. 

If  the  distances  are  too  great  for  boosters,  inverted  con- 
verters in  the  generating  station  are  used  to  change  from 
direct  current  to  alternating  current;  the  alternating 
current  is  sent  by  step-up  and  step-down  transformers  to 
the  substation  and  changed  to  direct  current  by  rotary 
converters. 

If  a  considerable  amount  of  power  is  required  at  a  dis- 
tance, it  is  more  convenient  at  the  generating  station  to  use, 
instead  of  inverted  converters,  double  current  generators, 
that  is,  generators  having  commutator  and  collector  rings. 


REGULATION  AND  CONTROL  111 

If  most  of  the  power  is  used  at  a  distance,  alternating- 
current  generators  are  used  with  rotary  converters  and  fre- 
quently one  converter  substation  is  located  in  the  generat- 
ing station. 

Inverted  converters  and  double  current  generators  are 
now  used  very  little,  since  usually  the  systems  are  now  so 
large  as  to  require  most  of  the  power  at  a  distance,  and 
therefore  alternating-current  generators  are  used. 

Many  big  systems  have  advanced  from  direct-current 
generators,  through  inverted  converters  and  double  cur- 
rent generators,  to  the  present  alternators  feeding  con- 
verter substations. 

B.  LOCAL  ALTERNATING-CURRENT  SYSTEMS 

Generator  Regulation. — First. — Close  inherent  regula- 
tion. 

This  is  secured  by  low  armature  reaction  and  high 
saturation  so  that  the  voltage  does  not  vary  much  with  the 
load. 

Advantages — 

Simple,  requiring  no  additional  apparatus,  etc. 

Instantaneous. 

Disadvantages — 

Larger  and  more  expensive  generators  and  when  of  very 
close  regulation,  more  difficult  to  run  in  parallel. 

Second. — Rectifying  Commutator. 

The  main  current  goes  over  a  commutator,  is  rectified, 
and  the  rectified  current  sent  through  a  series  field.  This 
arrangement  is  not  used  any  more. 

Advantage — 

Permits  compounding  and  overcompounding  without 
any  elaborate  apparatus. 

Disadvantages — 


112  GENERAL  LECTURES 

Only  limited  power  can  be  rectified,  therefore  suitable 
only  for  smaller  machines. 

Compound*  correctly  only  for  constant  power  factor; 
that  is,  if  compounded  for  non-inductive  load,  the  voltage 
drops  on  inductive  load,  since  inductive  load  requires  a 
greater  field  excitation  than  non-inductive  load. 

Brushes  have  to  be  shifted  with  change  of  power  factor, 
that  is,  change  from  motor  load  to  lighting  load,  etc.; 
otherwise  commutator  sparks  badly. 

These  machines  therefore  were  good  in  the  early  days 
when  all  the  load  was  lighting  load,  but  are  unsuited  at 
present  for  mixed  load. 

Third. — Potential  Regulator. 

Tirrill  Regulator. — Rheostat  in  exciter  field  so  large  that 
when  in  circuit  the  excitation  is  the  lowest,  and  that  when 
short-circuited  the  excitation  is  the  highest  ever  required. 

A  potential  magnet  in  the  alternator  circuit  operates  a 
contact-maker  which  continuously  cuts  the  resistance  in 
and  out  again,  so  that  the  contact-maker  is  never  at  rest, 
but  always  cuts  in  and  out,  and  the  average  field  excitation 
of  the  exciter  is  between  maximum  and  minimum. 

If  the  voltage  tends  to  drop,  the  contact  remains  a  shorter 
time  on  the  low  than  on  the  high  position,  and  so  raises 
excitation;  if  the  voltage  tends  to  rise,  the  contact-maker 
remains  a  shorter  time  on  the  high  than  on  the  low  posi- 
tion, and  so  lowers  excitation. 

Advantages — 

Very  simple. 

Can  be  applied  to  any  alternator  and  requires  no  special 
readjustment. 

Disadvantages — 

Additional  device  which  requires  some  attention  and 
adjustment. 


REGULATION  AND  CONTROL  113 

In  very  large  machines  often  no  regulating  device  is 
used  but  hand  control  of  the  field  rheostat,  since  in  such 
large  machines  the  load  only  varies  slowly  and  never 
changes  much,  as  for  reasons  of  economy  the  machines 
are  run  full  load;  with  the  change  of  load,  machines  are 
shut  down  or  started  up. 

Synchronous  Motors  and  Converters. 

In  an  alternating-current  system  or  part  of  the  system 
containing  large  synchronous  motors  or  converters  the 
voltage  can  be  controlled  by  varying  the  motor  or  converter 
field  in  the  same  way  as  with  alternators,  that  is,  by  Thrill 
regulator  or  commutator  and  series  field,  etc. 

Potential  Regulator •&__ 

(a)  Compensator  regulator. 

With  step-up  or  step-down  transformers  the  voltage  can 
be  regulated  by  having  different  taps  brought  out  of  the 
transformer  winding  and  so  get  different  voltages  by  means 
of  a  dial  switch.  Where  no  transformers  are  used  an  auto- 
transformer  with  different  voltage  taps  gives  the  same  results. 

The  taps  can  be  brought  out  in  the  primary  or  in  the 
secondary,  whichever  is  the  most  convenient:  in  the 
secondary,  if  the  primary  is  of  very  high  voltage;  in  the 
primary,  if  the  secondary  is  of  very  low  voltage  and  large 
current. 

Advantages — 

Simplest,  cheapest  and  most  efficient. 

Disadvantages — 

Step-by-step  variation. 

(b)  Induction  regulator. 

Built  like  induction  motors  with  stationary  primary  in 
shunt  and  movable  secondary  in  series  to  the  line. 

By  moving  the  secondary  the  voltage  varies  from  lower- 
ing to  raising. 

8 


114 


GENERAL  LECTURES 


Induction  regulators  are  usually  three-phase  and  of 
larger  sizes  for  rotary  converters  in  lighting  systems. 

When  single-phase,  the  stationary  member  contains  a 
short-circuitea  coil  at  right  angles  to  the  primary.  In  the 
neutral  position  this  coil  acts  as  short-circuited  secondary  to 
the  secondary  coil,  and  so  reduces  its  self-induction. 

Advantages — 

Perfectly  uniform  variation  and  considerable  inductance 
which  is  of  advantage  for  rotary  converters. 


FIG.  26. — Magneto  regulator. 

Disadvantages — 

High  cost. 

(c)  Magneto  regulator. 

A  stationary  primary  coil  is  in  shunt  and  a  stationary 
secondary  coil  is  in  series  and  at  right  angles  to  the  primary ; 
an  iron  shuttle  moves  inside  of  the  coils  and  so  turns  the 
magnetism  of  the  primary  coil  into  the  secondary  coil 
either  one  way  or  the  other. 

On  the  dotted  position  the  primary  sends  the  magnetism 


REGULATION  AND  CONTROL  115 

through  the  secondary  in  opposite  direction  as  in  the  drawn 
position,  in  Fig.  26. 

Advantage — 

Uniform  variation. 

Disadvantage — 

More  expensive  than  compensator  regulator. 

C.  GENERAL  POWER-GENERATING  SYSTEMS 

With  the  advances  of  electrical  engineering,  electric 
power  generation  for  all  purposes,  lighting,  domestic 
and  industrial  power,  railroading,  etc.,  is  more  and  more 
being  centralized  in  huge  three-phase  high-voltage  steam- 
turbine  stations,  often  interconnected  with  hydraulic 
stations.  In  these  vast  stations,  of  total  capacities  of 
hundred  thousands  of  kilowatts  connected  to  the  busbars, 
the  regulation  of  the  generators  has  ceased  to  be  of  any 
moment  in  the  voltage  control,  as  the  greatest  sudden 
change  of  load,  which  may  normally  be  expected,  is  too 
small  to  affect  the  busbar  voltage  of  such  a  system. 

The  generators  thus  are  usually  operated  with  ex- 
citers controlled  by  Tirrill  regulators  for  constant  voltage, 
and  the  serious  problem  of  such  systems  is  not  the  voltage 
regulation,  but  is  the  power  limitation,  is  to  reduce  the 
amount  of  power,  which  in  case  of  accident  can  be  let 
loose  at  any  point  of  the  system,  to  a  value  which  can  be 
controlled  without  serious  danger  of  self-destruction  of 
the  system.  The  solution  hereof  has  been  found  in  the 
extensive  use  of  reactances  in  the  generator  leads,  in  the 
busbars  and  often  in  the  feeders,  together  with  a  generator 
design  securing  the  highest  possible  internal  reactance. 
That  is,  in  this  case,  not  the  close  regulation  of  low  re- 
actance, as  desired  in  the  small  isolated  generators  of  old, 
is  aimed  at,  but  the  reverse  is  made  necessary  by  the  safety 
of  the  system :  high  reactance  and  limitation  of  the  power. 


ELEVENTH  LECTURE 
LIGHTNING  PROTECTION 

When  the  first  telegraph  circuits  were  strung  across  the 
country,  lightning  protection  became  necessary,  and  was 
given  to  these  circuits  at  the  station  by  connecting  spark 
gaps  between  the  circuit  conductors  and  the  ground. 

When,  however,  electric  light  and  power  circuits  made 
their  appearance,  this  protection  against  lightning  by  a 
simple  small  spark  gap  to  ground  became  insufficient,  and 
this  additional  problem  arose:  to  open  the  short-circuit 
of  the  machine  current,  which  resulted  from  and  followed 
the  lightning  discharge. 

This  problem  of  opening  the  circuit  after  the  discharge 
was  solved  by  the  magnetic  blowout,  which  is  still  used  to  a 
large  extent  on  500- volt  railway  circuits;  by  the  horn  gap 
arrester — a  gap  between  two  horn-shaped  terminals,  be- 
tween which  the  arc  rises,  and  so  lengthens  itself  until  it 
blows  out;  and  later  on,  for  alternating  current,  the  multi- 
gap  between  non-arcing  metal  cylinders,  a  number  of  small 
spark  gaps  in  series  with  each  other,  between  line  and 
ground,  over  which  the  lightning  discharges  to  ground — 
the  machine  current  following  as  arc,  but  stopped  at  the 
end  of  the  half  wave  of  alternating  current,  and  not  start- 
ing at  the  next  half  wave,  due  to  the  property  of  these 
"non-arcing"  metals  (usually  zinc-copper  alloys),  to  carry 
an  arc  in  one  direction,  but  requiring  an  extremely  high 
voltage  to  start  a  reverse  arc. 

These  lightning  arresters  operated  satisfactorily  with  the 

smaller  machines  and  circuits  of  limited  power  used  in  the 

116 


LIGHTNING  PROTECTION  117 

earlier  days,  but  when  large  machines  of  close  regulation, 
and  therefore  of  very  large  momentary  overload  capacity 
were  introduced,  and  a  number  of  such  machines  operated 
in  multiple,  these  lightning  arresters  became  insufficient: 
the  machine  current  following  the  lightning  discharge  fre- 
quently was  so  enormous  that  the  circuit  did  not  open 
at  the  end  of  the  half  wave,  but  the  arrester  held  an  arc 
and  burned  up. 

Furthermore,  the  introduction  of  synchronous  motors, 
and  of  parallel  operation  of  generators,  made  it  essential 
that  the  lightning  arrester  should  open  again  instantly  after 
discharge.  For,  if  the  short-circuit  current  over  the 
arrester  lasted  for  any  appreciable  time:  a  few  seconds, 
synchronous  motors  and  converters  dropped  out  of  step, 
the  generators  broke  their  synchronism,  and  the  system 
in  this  way  would  be  shut  down.  The  horn  gap  arrester, 
in  which  the  arc  rises  between  horn-shaped  terminals,  and 
by  lengthening,  blows  itself  out,  therefore  became  un- 
suitable for  general  service;  since  without  series  resistance, 
the  short-circuiting  arc  lasted  too  long  for  synchronous 
apparatus  to  remain  in  step,  and  with  series  resistance 
reducing  the  current  so  as  not  to  affect  synchronous  ma- 
chines, it  failed  to  protect  under  severe  conditions.  Thus 
it  has  been  relegated  for  use  as  an  emergency  arrester  on 
some  overhead  lines,  to  operate  only  when  a  shutdown  is 
unavoidable,  and  as  auxiliary  to  other  arresters. 

To  limit  the  machine  current  which  followed  the  light- 
ning discharge,  and  so  enable  the  lightning  arrester  to  open 
the  discharge  circuit,  series  resistance  was  introduced  in  the 
arrester.  Series  resistance,  however,  also  limited  the  dis- 
charge current,  and  with  very  heavy  discharges,  such 
lightning  arresters  with  series  resistance  failed  to  protect 
the  circuits,  that  is,  failed  to  discharge  the  abnormal  vol- 


118  GENERAL  LECTURES 

tage  without  destructive  pressure  rise.  This  difficulty  was 
solved  by  the  introduction  of  shunted  resistances,  that  is, 
resistances  shunting  a  part  of  the  spark  gaps.  All  the 
minor  discharges  then  pass  over  the  resistances  and  the 
unshunted  spark  gaps,  the  resistance  assisting  in  opening 
the  machine  circuit  after  the  discharge.  Very  heavy  dis- 
charges pass  over  all  the  spark  gaps,  as  a  path  without  re- 
sistance, but  those  spark  gaps  which  are  shunted  by  the 
resistance,  open  after  the  discharge;  the  machine  current, 
after  the  first  discharge,  therefore  is  deflected  over  the  re- 
sistances, limited  thereby;  and  the  circuit  so  finally  opened 
by  the  unshunted  spark  gaps. 

With  the  change  in  the  character,  size  and  power  of 
electric  circuits,  the  problem  of  their  protection  against 
lightning  thus  also  changed  and  became  far  more  serious 
and  difficult.  Other  forms  of  lightning,  which  did  not 
exist  in  the  small  electric  circuits  of  early  days,  also  made 
their  appearance,  and  protection  now  is  required  not  only 
against  the  damage  threatened  by  atmospheric  lightning, 
but  also  against  " lightning"  originating  in  the  circuits: 
so  called  "internal  lightning,"  which  is  frequently  far 
more  dangerous  than  the  disturbances  caused  by  thunder 
storms. 

Under  lightning  in  its  broadest  sense  we  now  understand 
all  the  phenomena  of  electric  power  when  beyond  control. 

Electric  power,  when  getting  beyond  control  may  mean 
excessive  currents,  or  excessive  voltages,  or  excessive 
suddenness:  high  frequency  and  steep  wave  fronts  of  im- 
pulses. Excessive  currents  are  usually  of  serious  moment 
only  in  systems  of  very  high  power :  since  the  damage  done 
by  excessive  currents  is  usually  due  to  heating,  and  even 
very  excessive  currents  require  an  appreciable  time  before 
producing  dangerous  temperatures,  usually  circuit-breakers, 


LIGHTNING  PROTECTION  119 

automatic  cutouts,  etc.,  can  take  care  of  excessive  currents, 
and  such  currents  produce  damage  only  in  those  instances 
where  they  occur  at  the  moment  of  opening  or  closing  a 
switch,  by  burning  contacts,  or  where  the  mechanical  forces 
exerted  by  them  are  dangerously  large,  as  with  the  short- 
circuit  currents  of  the  modern  huge  turbo-generators,  and 
in  general  in  systems  of  high  power  concentration. 

Excessive  voltage  is  practically  instantaneous  in  its 
action,  and  the  problem  of  lightning  protection  therefore 
is  essentially  that  of  protecting  against  excessive  voltages. 

The  performance  of  the  lightning  arrester  on  an  electric 
circuit  is  analogous  to  that  of  the  safety  valve  on  the  steam 
boiler,  that  is,  to  protect  against  dangerous  pressures— 
whether  steam  pressure  or  electric  pressure — by  opening  a 
discharge  path  as  soon  as  the  pressure  approaches  the 
danger  limit.  Therefore  absolute  reliability  is  required 
in  its  operation,  and  discharge  with  as  little  shock  as 
possible,  but  over  a  path  amply  large  to  discharge  prac- 
tically unlimited  power  without  dangerous  pressure  rise. 

However,  the  causes  of  excessive  pressures,  and  the  forms 
which  such  pressures  may  assume,  are  so  much  more  varied 
in  electric  circuits  than  with  steam  pressures,  that  the 
design  of  perfectly  satisfactory  lightning  arresters  has  been 
a  far  more  difficult  problem  than  the  design  of  the  steam 
safety  valve. 

Such  excessive  pressures  may  enter  the  electric  circuit 
from  the  outside  by  atmospheric  disturbances  as  lightning, 
or  may  originate  in  the  circuit. 

Excessive  pressures  in  electric  circuits  may  be  single 
peaks  of  pressure,  or  "strokes"  or  discharges,  or  multiple 
strokes;  that  is,  several  strokes  following  each  other  in 
rapid  succession,  with  intervals  from  a  small  fraction  of  a 
second  to  a  few  seconds,  or  such  excessive  pressures  may 


120  GENERAL  LECTURES 

be  practically  continuous,  the  strokes  following  each  other 
in  rapid  succession,  thousands  per  second,  sometimes  for 
hours. 

Atmospheric  disturbances,  as  cloud  lightning,  usually 
give  single  strokes,  but  quite  frequently  multiple  strokes,  as 
has  been  shown  by  the  oscillograms  secured  of  such  light- 
ning discharges  from  transmission  lines.  Any  lightning 
arrester  to  protect  the  system  must  therefore  be  operative 
again  immediately  after  the  discharge,  since  very  often  a 
second  and  a  third  discharge  follows  immediately  after  the 
discharge  within  a  second  or  less. 

Continuous  discharges,  or  recurrent  surges  (lightning 
lasting  continuously  for  long  periods  of  time  with  thousands 
of  high-voltage  peaks  per  second),  mainly  originate  in  the 
circuits:  by  an  arcing  ground,  spark  discharge  over  broken 
insulators,  faults  in  cables,  etc.  However,  this  fault  in 
the  circuit  frequently  is  caused  by  a  lightning  discharge,  for 
instance  by  a  flashing  over  of  an  insulator,  so  that  lightning 
may  be  the  ultimate  cause  of  the  surge.  These  phenomena, 
which  have  made  their  appearance  only  with  the  develop- 
ment of  the  modern  high-power  high-voltage  electric 
systems,  become  of  increasing  severity  and  danger  with  the 
increase  in  size  and  power  of  electric  systems. 

Single  strokes  and  multiple  strokes,  that  is,  all  the  dis- 
turbances directly  due  to  atmospheric  electricity,  as  cloud 
lightning,  are  safely  taken  care  of  by  the  modern  multi-gap 
lightning  arrester.  In  its  usual  form  for  high  alternating 
voltages,  it  comprises  a  large  number  of  spark  gaps,  con- 
nected between  line  and  ground,  and  shunted  by  re- 
sistances of  different  sizes,  as  shown  in  Fig.  27,  in  such 
manner  that  a  high  pressure  discharge  of  very  low  quantity, 
as  the  gradual  accumulation  of  static  charge  on  the  system, 
discharges  over  a  path  of  very  high  resistance  RI,  and  so 


LIGHTNING  PROTECTION 


121 


1 

1, 

n, 

A  ^i  J 

o         A 

z  °    ^ 
o 

o    U 

8-T 

8     4 

o        V 

0 

o 
o 
o 
o 

O,,.  ,.     . 

o 
o 
o 
o 

JL 

G 

FIG.  27 — High-voltage  multi-gap  lightning  arrester. 


122 


GENERAL  LECTURES 


discharges  inappreciably  and  even  frequently  invisibly. 
A  disturbance  of  somewhat  higher  power  finds  a  discharge 
path  of  moderate  resistance  RZt  and  so  discharges  with 
moderate  current,  that  is,  without  shock  on  the  system; 
while  a  high  power  disturbance  finds  a  discharge  path 


>ooooooo- 

1 

qp 

L 

L-O 


i 


FIG.  28. — Multi-gap  lightning  arrester. 

over  a  low  resistance  Rs,  and,  if  of  very  great  power,  even 
over  a  path  of  zero  resistance,  Z.  On  medium  voltage, 
commonly  only  two  resistances  are  used,  one  high  and  one 
moderately  low,  as  shown  by  the  diagram  of  a  2000- volt 
multi-gap  arrester,  Fig.  28,  and  on  voltages  of  2300  to 


LIGHTNING  PROTECTION  123 

10,000,  commonly  a  single  resistance  is  used,  which  is 
partly  in  shunt,  partly  in  series  to  the  gaps.  Gaps  and 
resistance  are  enclosed  in  a  closed  tube,  so  as  to  get  the 
assistance  of  the  pressure  created  by  the  discharge,  for 
opening  the  circuit.  This  type  of  the  arrester,  therefore, 
is  called  the  Compression  Chamber  Arrester.  It  is  gener- 
ally used  for  primary  alternating-current  distribution 
circuits,  and,  as  stated,  is  a  type  of  multi-gap  arrester. 

The  resistance  of  the  discharge  path  of  the  present  multi- 
gap  arrester  therefore  is  approximately  inversely  propor- 
tional to  the  volume  of  the  discharge.  This  is  an  essential 
and  important  feature.  Occasionally  discharges  of  such 
large  volume  occur,  as  to  require  a  discharge  path  of  no 
resistance,  as  any  resistance  would  not  allow  a  sufficient 
discharge  to  keep  the  voltage  within  safe  limits.  At  the 
same  time  the  discharge  should  not  occur  over  a  path  with- 
out a  resistance  or  of  very  low  resistance,  except  when 
necessary,  since  the  momentary  short-circuit — that  is, 
the  short-circuit  for  a  part  of  the  half  wave — of  a  re- 
sistanceless  discharge  is  a  severe  shock  on  the  system,  which 
must  be  avoided  wherever  permissible. 

This  type  of  lightning  arrester  takes  care  of  single  dis- 
charges and  of  multiple  discharges,  no  matter  how  fre- 
quently they  occur  or  how  rapidly  they  follow  each  other, 
with  the  minimum  possible  shock  on  the  system.  It  can- 
not take  care,  however,  of  continuous  lightning — those 
disturbances,  mainly  originating  in  the  system,  where  the 
voltage  remains  excessive  continuously  (or  rather  rises 
thousands  of  times  per  second  to  excessive  values),  and 
for  long  times.  With  such  a  recurring  surge,  the  multi- 
gap  arrester  would  discharge  continuously  in  protecting 
the  system,  until  it  destroys  itself  by  the  excessive  power 
of  the  continuously  succeeding  discharges. 


124  GENERAL  LECTURES 

Where  such  continuous  lightning  may  occur  frequently, 
as  in  large  high-power  systems,  and  in  all  high-voltage 
transmission  lines,  due  to  the  line  capacity,  and  the  system 
requires  protection  against  them,  a  type  of  lightning  ar- 
rester which  can  discharge  continuously,  at  least  for  a 
considerable  time,  without  self-destruction,  is  necessary. 
The  lightning  arrester  which  is  capable  of  doing  this,  is 
the  electrolytic,  or  aluminum  arrester.  In  its  usual  form 
(cone  or  disc  type)  it  comprises  a  series  of  cone-shaped 
aluminum  cells,  connected  between  line  and  ground  through 
a  spark  gap.  As  soon  as  the  voltage  of  the  system  rises 
above  normal,  by  the  value  for  which  the  spark  gap  is 
set,  a  discharge  takes  place  through  the  aluminum  cells, 
over  a  path  of  practically  no  resistance;  but  the  volume 
of  the  discharge  which  passes,  is  not  that  given  by  the 
voltage  on  the  system,  but  is  merely  that  due  to  the  excess 
voltage  over  the  normal,  since  the  normal  voltage  is  held 
back  by  the  counter  e.m.f.  of  the  aluminum  cells.  As  a 
result — with  strokes  following  each  other,  thousands  per 
second,  that  is,  with  a  recurrent  surge — the  aluminum 
arrester  discharges  continuously;  but  it  can  stand  the 
continuous  discharge  for  half  an  hour  or  more  without 
damage,  since  it  does  not  carry  the  short-circuit  current 
of  the  system,  but  merely  the  short-circuit  current  of  the 
excess  voltage,  and  so  protects  the  circuit  against  con- 
tinuous lightning  for  a  sufficiently  long  time,  until  the 
cause  of  the  high  voltage  can  be  found  and  eliminated. 

The  same  characteristics  as  that  of  the  aluminum 
arrester,  is  given  by  the  Oxide  Film  Arrester.  This  differs 
from  the  aluminum  arrester,  in  that  it  uses  no  liquid,  and 
requires  no  charging. 

Even  the  cone  type  of  aluminum  arrester  discharges  with 
a  slight  shock  on  the  system,  as  the  voltage  must  rise  to 


LIGHTNING  PROTECTION  125 

the  value  of  the  spark  gap,  before  the  discharge  begins,  and 
an  arrester  having  no  spark  gap,  would  therefore  be  very- 
desirable,  especially  in  systems,  in  which  even  a  small 
voltage  shock  is  objectionable,  as  mainly  in  large  under- 
ground cable  systems. 

Of  other  forms  of  lightning  arresters,  the  magnetic  blow- 
out 5 oo- volt  railway  arrester  is  still  in  use  to  a  large  extent, 
but  is  beginning  to  be  superseded  by  the  aluminum  cell. 
The  multi-gap,  being  based  on  the  non-arcing  or  rectifying 
property  of  the  metal  cylinders  which  exists  only  with 
alternating  current,  is  not  suitable  for  direct-current 
circuits.  In  arc-light  circuits,  that  is,  constant-current 
circuits,  horn  gap  arresters  with  series  resistance  are 
generally  used,  especially  on  direct-current  arc  circuits, 
in  which  the  multi-gap  is  not  permissible.  In  such  cir- 
cuits of  limited  current,  and  very  high  inductance,  the 
series  resistance  is  not  objectionable.  Otherwise  the  horn 
gap  arrester  is  still  occasionally  used  outdoors  as  emergency 
arrester  on  transmission  lines,  set  for  a  much  higher 
discharge  voltage  than  the  station  arrester,  and  then 
preferably  without  series  resistance,  but  in  such  use,  is  a 
serious  menace  to  the  system,  due  to  the  possibility  of  the 
arc  discharge  setting  off  destructive  oscillations. 

Horn  gaps  with  series  resistance  are  to  some  extent 
used  on  transmission  lines  as  a  cheap  form  of  arresters, 
in  smaller  substations,  branch  lines,  etc.,  where  the  cost 
of  apparatus  is  not  sufficient  to  justify  the  aluminum  ar- 
rester. Such  arrester  naturally  gives  partial  protection 
only,  against  lightning  disturbances  of  moderate  power. 

Protection  of  electric  circuits  is  required  against  excess 
currents,  against  excess  voltages,  and  against  excess 
frequencies,  including  impulses  of  steep  wave  front. 

The  protection  against  excess  currents  is  afforded  by 


126  GENERAL  LECTURES 

fuses,  circuit-breakers  and  in  general  by  limiting  the  power 
which  may  be  concentrated  at  any  point  of  the  system, 
and  in  the  huge  modern  electric  power-generating  systems, 
the  judicious  use  of  reactance  in  generators,  busbars  and 
feeders  has  solved  the  problem  of  limiting  the  possible 
power  concentration  to  values  within  the  control  of  auto- 
matic circuit-breakers,  without  interfering  with  the  parallel 
operation  of  the  entire  system  and  without  limiting  the 
size  of  the  system. 

Overvoltage  protection  is  afforded  by  the  lightning 
arresters,  as  discussed  above. 

Against  high  frequency  and  steep  wave  fronts,  such 
lightning  arresters  can  obviously  protect  only,  if  the  high 
frequency  is  of  such  high  voltage  as  to  discharge  over  the 
lightning  arrester;  but  where  this  is  not  the  case,  that  is, 
where  the  high-frequency  voltage  is  not  sufficient  to  dis- 
charge over  the  spark  gap  of  the  lightning  arrester,  the 
latter  obviously  cannot  protect. 

With  the  increasing  size  of  the  electric  systems,  and  their 
increasing  electrostatic  capacity,  in  cables,  high-potential 
overhead  lines,  etc.,  high-frequency  phenomena  or  impulses 
of  steep  wave  front  are,  however,  becoming  increasingly 
frequent,  are  indeed  the  most  serious  disturbances  affecting 
such  electric  systems. 

While  the  voltage  of  the  high-frequency  disturbance 
may  not  be  sufficiently  high  to  damage  the  insulation 
between  the  electric  circuit  and  the  ground,  the  serious 
danger  of  high  frequency  is  the  backing  up  of  voltage  across 
inductive  parts  of  the  circuit,  such  as  transformer  and 
generator  windings,  especially  the  end  turns  near  the 
terminals,  which  receive  the  high  frequency;  series  coils  of 
voltage  regulators ;  current  transformers,  etc.  The  destruc- 
tion caused  by  high  frequency  then  is  a  local  breakdown 


LIGHTNING  PROTECTION  127 

between  turns  or  coils,  resulting  in  a  short-circuit  thereof 
and  frequently  leading  to  the  destruction  of  the  apparatus 
by  the  heat  of  the  excessive  local  short-circuit  current. 

Inductance  exerts  a  barrier  action  against  high  frequency, 
and,  when  located  at  the  entrance  from  the  line  into  the 
station,  more  or  less  keeps  high  frequency  out  of  the  sta- 
tion, by  reflecting  it  back  into  the  line.  It  thus  exerts  a 
protecting  action  against  high  frequency  from  the  line, 
but  at  the  same  time  becomes  a  serious  danger  in  case  of 
high  frequency  originating  in  the  station,  as  by  switching: 
it  then  reflects  the  high  frequency  back  into  the  station 
instead  of  allowing  it  to  pass  into  the  line  where  it  is  dissi- 
pated in  the  line  resistance.  However,  such  barrier  in- 
ductances usually  are  economically  feasible  only  for  very 
high  frequency  such  as  produced  by  lightning,  of  many 
hundred  thousands  of  cycles.  At  lower  frequencies,  for 
instance,  at  the  frequencies  met  in  the  stationary  waves 
or  cumulative  oscillations  of  high-potential  high-power 
transformers — of  20,000  to  100,000  cycles — the  required 
amount  of  inductance  becomes  so  large  as  to  be  economic- 
ally impracticable. 

Capacity  shunting  the  circuit  is  effective  in  shunting 
high  frequency,  and  absorbs  the  initial  steep  wave  front 
of  an  impulse,  which  by  its  high-voltage  gradient  in  induct- 
ance is  so  destructive,  and  thereby  flattens  out  the  wave 
front. 

Resistance  absorbs  the  high  frequency  or  impulse  energy. 

The  most  effective  protection  against  high-frequency 
oscillations  and  against  sudden  impulses  of  steep  wave 
front,  therefore,  is  afforded  by  a  combination  of  capacity 
and  resistance:  the  resistance,  in  shunt  to  the  circuit, 
absorbs  the  high-frequency  energy,  and  a  capacity  in  series 
to  the  resistance  keeps  the  low-frequency  machine  current 


128  GENERAL  LECTURES 

from  flowing  through  the  resistance  shunt,  while  it  allows 
the  high-frequency  to  pass  unobstructed.  Thus  such  com- 
bination of  resistance  and  capacity  acts  as  effective  high- 
frequency  absorber,  and  as  such  is  extensively  used,  alone 
or  in  combination  with  the  barrier  action  of  an  inductance. 
In  flattening  the  wave  front  and  thereby  protecting 
apparatus  from  ''spilling  over"  by  the  sudden  voltages  of 
a  very  steep  wave  front,  capacity  without  any  resistance 
in  series  thereto  is  most  effective,  while  for  high-frequency 
energy  dissipation,  resistance  is  required  in  addition  to  the 
capacity. 


TWELFTH  LECTURE 
ELECTRIC  RAILWAY 

TRAIN  CHARACTERISTICS 

The  performance  of  a  railway  consists  of  acceleration, 
motion  and  retardation,  that  is,  starting,  running  and 
stopping. 

The  characteristics  which  the  railway  motor  must 
possess,  are: 

1.  Reliability. 

2.  Limited  available  space,  which  permits  less  margin 
in  the  design,  so  that  the  railway  motor  runs  at  a  higher 
temperature,  and  has  a  shorter  life,  than  other  electrical 
apparatus.     The  rating  of  a  railway  motor  is,  therefore, 
entirely  determined  by  its  heating.     That  is,  the  rating  of 
a  railway  motor  is  that  output  which  it  can  carry  without 
its  temperature  exceeding  the  danger  limit.     The  highest 
possible  efficiency  is,  therefore,  aimed  at,  not  so  much  for 
the  purpose  of  saving  a  few  per  cent,  of  power,  but  because 
the  power  lost  produces  heat  and  so  reduces  the  motor 
output. 

3.  Very  variable  demands  in  speed.     That  is,  the  motor 
must  give  a  wide  range  of  torque  and  speed  at  high  effi- 
ciency.    This  excludes  from  ordinary  railway  work  the 
shunt  motor  and  the  induction  motor,   unless  a  radical 
change  in  the  method  of  railroad  operation  is  accepted. 

The  power  consumed  in  acceleration  usually  is  many 
times  greater  than  when  running  at  constant  speed,  and 
where  acceleration  is  very  frequent,  as  in  rapid  transit 
service,  the  efficiency  of  acceleration  is,  therefore,  of  fore- 

9  129 


130  GENERAL  LECTURES 

most  importance,  while  in  cases  of  infrequent  stops,  as  in 
long-distance  and  interurban  lines,  the  time  of  acceleration 
is  so  small  a  part  of  the  total  running  time,  that  the  power 
consumed  during  acceleration  is  a  small  part  of  the  total 
power  consumption,  and  high  efficiency  of  acceleration  is, 
therefore,  of  less  importance. 

Typical  classes  of  railway  service  are: 

1.  Rapid  transit,  as  elevated  and  subway  roads  in  large 
cities. 

Characteristics  are  high  speeds  and  frequent  stops. 

2.  City  surface  lines,  that  is,  the  ordinary  trolley  car 
in  the  streets  of  a  city  or  town. 

.  Moderate  speeds,  frequent  stops,  and  running  at  vari- 
able speeds,  and  frequently  even  at  very  low  speeds,  are 
characteristic. 

3.  Suburban  and  interurban  lines.     That  is,  lines  lead- 
ing from  cities  into  suburbs  and  to  adjacent  cities,  through 
less  densely  populated  districts. 

Characteristics  are  less  frequent  stops,  varying  speeds, 
and  the  ability  to  run  at  fairly  high  speeds  as  well  as  low 
speeds. 

4.  Long-distance  and  trunk  line  railroading. 
Characteristics  are:  infrequent  stops,  high  speeds,  and  a 

speed  varying  with  the  load,  that  is,  with  the  profile  of  the 
road. 

5.  Special   classes   of   service,    as   mountain   roads   and 
elevators. 

Characteristics  are  fairly  constant  and  usually  moderate 
speed ;  a  constant  heavy  load,  so  that  the  power  of  accelera- 
tion is  not  so  much  in  excess  of  that  of  free  running;  and 
usually  frequent  stops.  This  is  the  class  of  work  which  can 
well  be  accomplished  by  a  constant-speed  motor,  as  the 
three-phase  induction  motor,  and  where  regeneration,  that 


ELECTRIC  RAILWAY  131 

is,  returning  power  as  generator,  on  the  down  grade,  is 
desirable. 

The  rate  of  acceleration  and  rate  of  retardation  is  limited 
only  by  the  comfort  of  the  passengers,  which  in  this  country 
permits  as  high  values  as  2  to  2^  miles  per  hour  per  second, 
that  is,  during  every  second  of  acceleration,  the  speed  in- 
creases at  the  rate  of  2  to  2^  miles  per  hour,  so  that  i 
second  after  starting  a  speed  of  2  to  2%  miles  per  hour,  5 
seconds  after  starting  a  speed  of  5  X  2  to  2%  =  10  to 
i2j£  miles  per  hour,  etc.,  is  reached. 

In  freight  service  by  electric  locomotive,  the  permissible 
acceleration  may  be  limited  by  the  traction,  that  is,  the 
slipping  of  the  drive  wheels  of  the  locomotive,  as  the 
locomotive  is  a  small  part  of  the  train  weight  only. 

Steam  trains  give  accelerations  of  J^  mile  per  hour  per 
second  and  less  with  heavy  trains,  due  to  the  lesser  maxi- 
mum power  of  the  steam  locomotive. 

Speed -time  Curves. — In  rapid  transit,  and  all  service 
where  stops  are  so  frequent  that  the  power  consumed  dur- 
ing acceleration  is  a  large  part  of  the  total  power,  the  speed- 
time  curves  are  of  foremost  importance,  that  is,  curves  of 
the  car-run,  plotted  with  the  time  as  abscissae,  and  the 
speed  as  ordinate. 

Choose  for  instance,  a  maximum  acceleration  and  maxi- 
mum braking  of  2  miles  per  hour  per  second,  and  assuming 
a  retardation  of  J4  mile  per  hour  per  second  by  friction 
(that  is,  assuming  that  the  car  slows  down  y±  mile  per 
second,  when  running  light  on  a  level  track) ;  if  then  the 
time  of  one  complete  run  between  two  stations  is  given 
equal  to  AB  in  Fig.  29,  the  simplest  type  of  run  consists  of 
constant  acceleration,  from  A  to  C,  on  the  line  Aa,  drawn 
under  a  slope  of  2  miles  per  hour  per  second;  at  C  the 
power  is  shut  off  and  the  car  coasts  on  the  slope  CD,  of  J-^ 


132 


GENERAL  LECTURES 


mile  per  hour  per  second,  until  at  D,  where  the  coasting 
line  cuts  the  braking  line  bB  (which  also  is  drawn  at  the 
slope  of  2  miles  per  hour  per  second) ,  the  brakes  are  applied 
and  the  car  comes  to  rest,  at  B.  As  the  distance  traveled  is 
speed  times  time,  the  area  ACDB  so  represents  the  distance 
traveled,  that  is,  the  distance  between  the  two  stations, 
and  all  speed-time  curves  of  the  same  type,  therefore,  must 
give  the  same  area.  During  acceleration,  energy  is  put 
into  the  car,  and  stored  by  its  momentum,  which  is  pro- 


5*J 


& 


FIG.  29. — Rapid  transit  speed-time  curve. 

portional  to  the  weight  of  the  car  and  the  square  of  the 
speed.  It  is,  therefore,  at  a  maximum  at  C.  A  part  of 
the  energy  represented  by  the  car  speed  is  consumed  dur- 
ing coasting  in  overcoming  the  friction;  the  rest  is  de- 
stroyed by  the  brakes.  Assuming,  as  approximation,  con- 
stant friction,  the  energy  consumed  by  the  car  friction  on 
the  track,  for  runs  of  the  same  distance,  is  constant,  and 
the  energy  destroyed  by  the  brakes  is  represented  by  the 
speed  at  the  point  B,  where  the  brakes  are  applied.  The 
lower,  therefore,  this  point  B  is,  the  less  power  is  destroyed 


ELECTRIC  RAILWAY  133 

by  the  brakes,  and  the  more  efficient  is  the  run.  More  ac- 
curately, by  prolonging  CD  to  E  so  that  area  DEG  =  BFG, 
the  area  ACEF  also  is  the  distance  between  the  stations, 
and  EF  so  would  be  the  speed  at  which  the  car  arrives  at 
the  next  station,  if  no  brakes  were  applied,  and  the  energy 
corresponding  thereto  has  to  be  destroyed  by  the  brakes; 
that  is,  represents  the  energy  lost  during  the  run,  and 
should  be  made  as  small  as  possible,  to  secure  efficiency. 

The  ratio  of  the  energy  used  for  carrying  the  car  across 
the  distance  between  the  stations — that  is,  energy  con- 
sumed by  track  friction,  (plus  energy  consumed  in  climb- 
ing grades,  where  such  exist)  to  the  total  energy  input, 
that  is,  track  friction  plus  energy  consumed  in  the  brakes, 
is  the  operation  efficiency  of  the  run. 

As  an  illustration,  a  number  of  such  runs,  for  constant 
time  of  the  run,  of  130  seconds,  and  constant  distance  be- 
tween the  stations,  that  is,  constant  area  of  the  speed- 
time  diagram,  are  plotted  in  Figs.  29  to  37. 

1.  Constant  acceleration  of  2  miles  per  hour  per  second, 
coasting  at  y±  mile  per  hour  per  second,  and  braking  at  2 
miles  per  hour  per  second.     Here  the  energy  consumed  by 
the  brakes  is  given  by  the  speed  EF  =  34.5  miles  per  hour, 
while  the  maximum  speed  reached  is  60  miles  per  hour.   • 

2.  Acceleration  and  retardation  at  2  miles  per  hour  per 
second.     Constant  speed  running  between  Fig.  30.     Com- 
pared with  i   (which  is  shown  in  30  in  dotted  lines),  the 
maximum  speed  is  slightly  reduced,  e.g.,  to  51  miles  per 
hour,  but  the  speed  of  application  of  the  brakes,  and  there- 
fore the  energy  lost  in  the  brakes,  is  increased.     That  is, 
running    at    constant    speed,    between    acceleration    and 
braking,    is   less   efficient   than   coasting   with   decreasing 
speed.     Besides  this,  at  the  low  power  required  for  constant 
speed  running,  the  motor  efficiency  usually  is  already  lower. 


134 


GENERAL  LECTURES 


It,  therefore,  is  uneconomical  to  keep  the  power  on  the 
motors  after  acceleration,  and  more  economical  to  con- 
tinue to  accelerate  until  a  sufficient  speed  is  reached  to  coast 
until  the  brakes  have  to  be  applied  for  the  next  station. 
Obviously,  this  is  not  possible  where  the  distance  between 
the  stations  is  so  great,  that  in  coasting  the  speed  would 
decrease  too  much  to  make  the  time,  and  so  applies  only 
to  the  case  of  runs  with  frequent  stops,  as  rapid  transit. 

3.  Constant  acceleration  of  i  mile  per  hour  per  second, 
braking  at  2  miles,  coasting  y±  mile.     Diagram  i  is  shown 


PIG.  30. — Speed-time  curve  with  constant  speed  running. 

in  the  same  Fig.  31,  for  comparison.  As  seen,  with  the 
lower  rate  of  acceleration,  the  maximum  speed  is  greater, 
and  the  lost  speed,  or  speed  EF,  which  is  destroyed  by  the 
brakes,  is  greater,  that  is,  the  efficiency  of  the  run  is  lower. 
4.  Constant  acceleration  and  braking  of  i  mile  per 
hour-  per  second,  coasting  at  y±  mile.  In  this  case,  the 
run  between  the  stations  cannot  be  made  in  130  seconds. 
For  comparison,  i  is  shown  dotted  in  Fig.  32.  Here  the 
maximum  speed  and  the  lost  speed  are  still  greater,  that  is, 


ELECTRIC  RAILWAY 


135 


the  efficiency  of  the  run  still  lower,  and  at  least  145  seconds 
are  required.     That  is,  the  higher  the  rate  of  acceleration 


B 


FIG.  31. — Slow  acceleration  speed-time  curve. 

and  of  braking,  the  less  is  the  maximum  speed  required, 
and  the  higher  the  operation  efficiency.     With  constant 


\\ 


FIG.  32. — Slow  acceleration  and  slow  braking  speed-time  curve. 

acceleration   up   to   the   maximum   speed,    the  operation, 
therefore,  is  the  more  efficient  the  higher  a  rate  of  accelera- 


136  GENERAL  LECTURES 

tion  and  of  braking  is  used.  While  very  rapid  acceleration 
requires  more  power  developed  by  the  motor  and  put  into 
the  car,  the  time  during  which  the  power  is  developed 
is  so  much  shorter,  that  the  energy  put  into  the  car,  or 
power  times  time  of  power  application,  is  less  than  with 
the  lower  rate  of  acceleration. 

The  highest  operation  efficiency,  in  the  case  of  fre- 
quent stops,  therefore,  is  produced  by  constant  accelera- 
tion at  the  highest  permissible  rate,  coasting  without 
power,  and  then  braking  at  the  highest  permissible  rate, 
as  given  by  i. 

During  acceleration  at  constant  rate,  A  to  (7,  the  motor, 
however,  runs  on  the  rheostat.  That  is,  at  all  speeds  be- 
low the  maximum,  to  produce  the  same  pull  as  at  the  maxi- 
mum speed  C,  the  motor  consumes  the  same  current  and  so 
the  same  power;  while  the  power  which  it  puts  into  the 
train  is  proportional  to  the  speed,  and  therefore  is  very 
low  at  low  speeds.  Or,  in  other  words,  the  motor  during 
constant  acceleration,  consumes  power  corresponding  to 
maximum  speed,  while  the  useful  power  corresponds  to 
the  average  speed,  which  during  AC  is  only  half  the  maxi- 
mum; and  so  only  half  the  available  power  is  put  into  the 
car,  the  other  half  being  wasted  in  the  resistance,  and  the 
motor  efficiency  during  constant  acceleration,  therefore,  must 
be  less  than  50  per  cent. 

Constant  acceleration  up  to  maximum  speed,  while  giv- 
ing the  best  operation  efficiency,  so  gives  a  very  poor  motor 
efficiency  and  thereby  low  total  efficiency  (the  total  effi- 
ciency being  the  ratio  of  the  useful  energy  to  the  total  en- 
ergy put  into  the  motors,  that  is,  it  is  operation  efficiency 
times  motor  efficiency). 

This  is  the  arrangement  necessary  for  a  constant-speed 
motor,  as  the  induction  motor ;  but  it  does  not  give  the  best 


ELECTRIC  RAILWAY 


137 


total  efficiency,  but  a  better  total  efficiency  is  produced  by 
accelerating  partly  on  the  motor  curve,  that  is,  at  a  decreas- 
ing rate.  This  sacrifices  some  operation  efficiency,  but 
increases  the  motor  efficiency  greatly  and  so,  if  not  carried 
too  far,  increases  the  total  efficiency. 

The  speed-time  curves  of  the  motor  are  shown  in  Fig.  33, 
and  the  current  consumption  is  also  plotted  in  this  figure. 
Acceleration  is  constant  from  A  to  M,  on  the  rheostat,  and 


FIG.  33. — Series  motor  speed-time  curve. 

at  constant-current  consumption,  from  M,  onward,  the 
acceleration  decreases,  first  slightly,  then  faster,  but  the 
current  also  decreases,  first  rapidly,  and  then  more  slowly; 
and  the  efficiency,  plotted  in  Fig.  33,  rises  from  o  per  cent, 
at  A,  to  90  per  cent,  at  M,  and  then  remains  approximately 
constant,  while  the  speed  still  increases. 

6.  This  gives  the  speed-time  curve  of  the  car,  Fig.  34, 
with  acceleration  on  the  motor  curve  and  with  maximum 
values  of  acceleration  and  braking  2,  the  coasting  value 


138 


GENERAL  LECTURES 


one-quarter ;  that  is,  the  same  as  i ,  and  i  is  shown  in  dotted 
lines  in  the  same  figure.  The  acceleration  is  constant,  on 
the  rheostat,  from  A  to  M;  at  M  the  rheostat  is  cut  out,  and 
the  acceleration  continues  on  the  motor  curve,  at  a  gradu- 
ally decreasing  rate,  until  at  C  the  power  is  shut  off  and 
the  car  coasts  until  the  brakes  are  applied.  The  area 
AMCDB,  representing  the  distance  between  the  stations, 
is  the  same  as  in  i ;  the  operation  efficiency  is  somewhat 


7k 


< 


/M 


FIG'  34. — Series  motor  speed-time  curve. 

lower,  but  the  total  current  consumption,  as  shown  by  the 
curves  of  current,  shown  together  with  the  speed- time 
curves,  is  much  less,  and  the  power  consumption,  therefore, 
is  less ;  that  is,  the  total  efficiency  is  higher. 

7.  Fig.  35  gives  another  speed-time  curve  in  which,  how- 
ever, the  motor  is  geared  for  too  low  a  speed;  so  the  motor 
curve  is  reached  too  early,  and  the  power  has  to  be  kept  on 
for  too  long  a  time,  to  make  the  run  in  time.  As  seen  from 


ELECTRIC  RAILWAY 


139 


the  current  curves,  here  the  loss  in  car  efficiency  by  the  de- 
creased acceleration  on  the  motor  curve  is  greater  than 
the  saving  in  motor  efficiency,  and  the  power  consumption 
by  the  motor  is  greater  than  that  without  running  on  the 
motor  curve. 

That  is,  the  total  efficiency  of  operation  is  increased  by 
doing  some  of  the  accelerating  on  the  motor  curve,  but 
may  be  impaired  again  by  carrying  this  too  far.  Usually 
the  rheostat  is  all  cut  out  and  the  acceleration  continues 
on  the  motor  curve,  from  about  half  speed  onward. 


B 


FIG.  35. — Inefficient  speed-time  curve. 

8.  During  the  first  half  of  the  acceleration  on  the  rheo- 
stat, when  more  than  half  the  voltage  is  consumed  in  the 
rheostat,  half  the  current  can  be  saved  by  connecting  two 
motors  in  series;  that  is,  by  series  parallel  control  on  the 
motors,  as  shown  in  Fig.  36.  If,  however,  the  series  con- 
nection of  motors  is  maintained  too  long,  as  shown  in 
Fig.  37>  so  that  the  Part  °f  tne  curve  SP  gets  too  long,  the 
average  rate  of  acceleration,  and  so  the  operation  efficiency, 
is  greatly  reduced.  That  is,  the  lost  area  becomes  so 
large,  that  the  speed  at  application  of  the  brakes,  and  so 


140 


GENERAL  LECTURES 


the   power   lost   in   brakes,    is    greatly   increased.     Series 
connection  of  motors,  for  efficient  acceleration,  therefore 


g 


FIG.  36. — Efficient  series- parallel  control. 


i   FIG.  37. — Inefficient  series-parallel  control. 

should  not  be  maintained  for  any  length  of  time  after  the 
rheostat  has  been  cut  out. 


ELECTRIC  RAILWAY  141 

In  series  parallel  control,  as  shown  in  Figs.  36  and  37, 
some  acceleration  occurs  on  the  motor  curve  in  series  con- 
nection. That  is,  AS  is  acceleration  on  the  rheostat,  in 
series  connection,  SP  acceleration  on  the  motor  curve; 
PM  on  the  rheostat  in  parallel  connection,  and  MC  on  the 
motor  curve  in  parallel  connection.  Compared  with  i, 
which  is  shown  dotted  in  37,  the  area  ASPMHC\  is  lost; 
and  so  the  equal  area  HCDDi,  has  to  be  gained,  giving  a 
higher  speed  of  application  of  the  brakes  D,  but  gaining 
power  more  than  the  increased  power  consumption  in  the 
brakes,  by  the  higher  motor  efficiency. 

CONCLUSION 

In  short-distance  runs  the  efficiency  is  highest  in  running 
on  series  parallel  control  as  much  as  possible  on  the  motor 
curve,  with  as  high  a  rate  of  average  acceleration  and  re- 
tardation as  possible,  and  coasting  between  acceleration 
and  retardation;  that  is,  not  keeping  the  power  on  longer 
than  necessary. 

The  longer  the  distance,  the  less  important  is  high  rate 
of  acceleration  and  retardation,  and  for  long-distance  run- 
ning the  rate  of  acceleration  and  retardation  is  of  little 
importance. 

Therefore,  speed-time  curves  are  specially  important  in 
rapid  transit  service,  and  in  general,  in  running  with 
frequent  stops. 

The  heating  of  the  motor  at  high  acceleration,  that  is, 
with  large  current,  is  less  than  with  low  acceleration,  that  is, 
smaller  current,  because  the  current  is  on  a  much  shorter 
time. 

Feeding  back  in  the  line  by  using  the  motors  as  genera- 
tors is  rarely  used  in  rapid  transit ;  because  with  an  efficient 
speed-time  curve,  using  coasting,  the  speed  when  putting 


142  GENERAL  LECTURES 

on  the  brakes  is  already  so  low  that  usually  not  enough 
power  can  be  saved  to  compensate  for  the  complication 
and  the  increased  heating  of  the  motors,  when  carrying 
current  also  in  stopping.  The  motors  are  occasionally 
used  as  brakes,  operating  as  generators  on  the  rheostat. 
This,  however,  puts  an  additional  heating  on  the  motors; 
and  is  therefore  not  much  used  in  this  country,  where  the 
highest  speed  which  the  motor  equipment  can  give  is  desired. 

In  long-distance  railroading,  however,  and  especially 
in  mountainous  regions,  feeding  back  offers  many  ad- 
vantages, not  only  in  saving  power,  but  in  saving  brakes, 
and  in  better  train  control  and  higher  safety,  and  methods 
of  feeding  power  back  on  down  grades,  of  " regeneration," 
are  increasingly  becoming  of  importance  in  long-distance 
railroading. 

With  induction  motors,  feeding  back  in  the  line  is 
simplest,  because  induction  motors  become  generators 
above  synchronism,  and  so  feed  back  when  running  down 
a  long  hill.  Therefore  on  mountain  railways,  induction 
motors  have  the  advantage. 

In  an  induction  motor  there  is  no  running  on  the  motor 
curve,  and  so  the  efficiency  of  acceleration  is  lower. 

Objection  to  the  series  motor  is  the  unlimited  speed;  that 
is,  when  running  light,  it  runs  away.  In  railroading  this 
is  no  objection,  because  the  motor  is  never  running  light 
and  somebody  is  always  in  control. 

In  elevator  work  the  series  motor  is  objectionable,  due 
to  the  unlimited  speed;  therefore,  a  limited-speed  motor  is 
necessary.  In  elevators  frequent  stops,  and  so  efficient 
acceleration  are  necessary;  therefore,  a  compound  motor 
is  best,  that  is,  a  motor  having  a  shunt  field  to  limit  the 
speed  and  a  series  field  (which  is  cut  out  after  starting)  to 
give  efficient  acceleration. 


THIRTEENTH  LECTURE 

ELECTRIC  RAILWAY:     MOTOR 
CHARACTERISTICS 

The  economy  of  operation  of  a  railway  system,  station, 
lines,  etc.,  decreases  and  the  amount  of  apparatus,  line 
copper,  etc.,  which  is  required,  increases  with  increasing 
fluctuations  of  load ;  the  best  economy  of  an  electric  system 
therefore  requires  as  small  a  power  fluctuation  as  possible. 

The  pull  required  of  the  railway  motor  during  accelera- 
tion, on  heavy  grades,  etc.,  is,  however,  many  times  greater 
than  in  free  running.  In  a  constant-speed  motor,  as  a 
direct-current  shunt  motor  or  an  alternating-current  induc- 
tion motor,  the  power  consumption  is  approximately 
proportional  to  the  torque  of  the  motor  and  thus  to  the 
drawbar  pull  that  is  given  by  it.  With  such  motors,  the 
fluctuation  of  power  consumption  would  thus  be  as  great 
as  the  fluctuation  of  pull  required.  In  a  varying-speed 
motor,  as  the  series  motor,  the  pull  increases  with  decreas- 
ing speed;  and  the  power  consumption,  which  is  approxi- 
mately proportional  to  pull  times  speed,  varies  less  than 
the  pull  of  the  motor.  The  fluctuation  of  load  produced 
in  the  circuit  by  a  series  motor  therefore  is  far  less  than  that 
produced  by  a  shunt  or  induction  motor — the  former 
economizing  power  at  high  pull  by  a  decrease  of  speed; 
the  series  motor  thus  gives  a  more  economical  utilization  of 
apparatus  and  lines  than  the  shunt  or  induction  motor, 
and  is  therefore  almost  exclusively  used. 

The  torque,  and  so  the  pull  produced  by  a  motor,  is 

approximately  proportional  to  the  field  magnetism  and  the 

143 


144 


GENERAL  LECTURES 


armature  current ;  that  is,  neglecting  the  losses  in  the  motor, 
or  assuming  100  per  cent,  efficiency,  the  torque  is  propor- 
tional to  the  product  of  magnetic  field  strength  and  arma- 
ture current. 

In  a  shunt  motor,  at  constant  supply  voltage  e,  the  field 
exciting  current,  and  thus  the  field  strength,  is  constant; 
and  the  torque,  when  neglecting  losses,  is  thus  proportional 
to  the  armature  current,  as  shown  by  the  curve  T0  in  Fig. 
38.  From  this  torque  is  subtracted  the  torque  consumed 


E&18 


FIG.  38. — Shunt  motor  characteristic. 

by  friction  losses,  core  loss,  etc.  (which,  at  approximately 
constant  speed  and  field  strength,  is  approximately  con- 
stant and  is  shown  by  the  curve  7\)  thus  giving  as  net 
torque  of  the  motor,  the  curve  T.  Neglecting  losses,  the 
speed  of  the  motor  would  be  constant,  as  given  by  line  S0, 
since  at  constant  field  strength,  to  consume  the  same  supply 
voltage  eot  the  armature  has  to  revolve  at  the  same  speed. 
As,  however,  with  increasing  load  and  therefore  increasing 
current,  the  voltage  available  for  the  rotation  of  the  arma- 


ELECTRIC  RAILWAY:  MOTOR  CHARACTERISTICS    145 


ture  decreases  by  the  ir  drop  in  the  armature,  as  shown  by 
the  curve  e\  at  constant  field  strength  the  speed  decreases 
in  the  same  proportion,  as  shown  by  the  curve  Si.  The 
field  strength,  however,  does  not  remain  perfectly  constant, 
but  with  increasing  load  the  field  magnetism  slightly  chan- 
ges: it  decreases  by  field  distortion  and  demagnetization, 
and  the  speed  therefore  increases  in  the  same  proportion, 
to  the  curve  S.  The  current  used  as  abscissae  in  Fig.  38  is 
the  armature  current.  The  total  current  consumed  by  the 


FIG.  39. — Shunt  motor  characteristic. 

motor  is,  however,  slightly  greater,  namely,  by  the  exciting 
current  i0\  and,  plotted  for  the  total  current  of  the  motor  as 
abscissae,  all  the  curves  in  Fig.  38  are  therefore  shifted  to 
the  right,  by  the  amount  of  i0,  as  shown  in  Fig.  39. 

If  in  the  shunt  motor,  the  supply  voltage  changes,  the 
field  strength,  which  depends  upon  the  supply  voltage,  also 
changes ;  it  decreases  with  a  decrease  of  the  supply  voltage, 
and  the  current  required  to  produce  the  same  torque  there- 
fore increases  in  the  same  proportion.  If  the  magnetic 

10 


146  GENERAL  LECTURES 

field  is  below  saturation,  the  field  strength  decreases  in 
proportion  to  the  decrease  of  supply  voltage,  and  the  cur- 
rent thus  increases  in  proportion  to  the  decrease  of  supply 
voltage,  while  the  speed  remains  the  same,  the  armature 
produces  the  lower  voltage  by  revolving  in  the  lower  field 
at  the  same  speed.  If  the  magnetic  field  is  highly  over- 
saturated  and  does  not  therefore  appreciably  change  with  a 
moderate  change  of  supply  voltage  and  so  of  field  current, 
the  armature  current  required  to  produce  the  same  torque 
also  does  not  appreciably  change  with  a  moderate  drop  of 
supply  voltage,  but  the  speed  decreases,  since  the  armature 
must  now  consume  less  voltage  in  the  same  field  strength. 

Depending  on  the  magnetic  saturation  of  the  field :  with 
a  decrease  of  the  supply  voltage  the  current  consumed  by 
the  shunt  motor  to  produce  the  same  torque,  therefore 
increases  the  more,  the  lower  the  saturation,  and  the  speed 
decreases  the  more,  the  higher  the  saturation. 

In  general,  a  drop  of  voltage  in  the  resistance  of  lines 
and  feeders  does  not  much  affect  the  speed  of  the  shunt 
motor,  but  increases  the  current  consumption,  thus  still 
further  increasing  the  drop  of  voltage;  so  that  in  a  shunt- 
motor  system,  lines  and  feeders  must  be  designed  for  a 
lower  drop  in  voltage  than  is  permissible  for  a  series  motor. 

The  induction  motor  in  its  characteristics  corresponds 
to  a  shunt  motor  with  undersaturated  field,  except  that  the 
effect  of  a  drop  of  voltage  is  still  more  severe;  as  not  only 
the  amount,  but  usually  the  lag  of  current  also  increases, 
thus  causing  more  drop  in  voltage;  and  the  maximum 
torque  of  the  motor  is  limited,  and  decreases  with  the  square 
of  the  voltage.  Hence,  while  in  a  series-motor  system  the 
lines  and  feeders  are  designed  for  the  average  load  or  aver- 
age voltage  drop  (and  practically  no  limit  exists  to  the 
permissible  maximum  voltage  drop),  with  an  induction 


ELECTRIC  RAILWAY:  MOTOR  CHARACTERISTICS    147 


motor,  the  maximum  permissible  voltage  drop  is  limited 
by  the  danger  of  stalling  the  motors. 


(.VRREN 


LkUAY. 


80 


^^ 


» 


10 


FIG.  40. — Series  motor  characteristic 

In  the  series  motor,  the  armature  current  passes  through 
the  field,   and   with  increasing  load   and   thus  increasing 


148  GENERAL  LECTURES 

current,  the  field  strength  also  increases;  the  torque  of  the 
motor  therefore  increases  in  a  greater  proportion  than 
the  current.  Neglecting  losses  and  saturation,  the  field 
strength  is  proportional  to  the  current ;  the  torque  being  pro- 
portional to  the  current  times  field  strength,  therefore  is 
proportional  to  the  square  of  the  current,  as  shown  by  the 
curve  T0  in  Fig.  40.  The  supply  voltage,  however,  has 
no  direct  effect  on  the  torque;  but  with  the  same  current 
consumption,  the  motor  gives  the  same  torque,  regardless 
of  the  supply  voltage.  The  speed,  at  constant  supply 
voltage,  changes  with  the  field  strength  and  thus  with  the 
current :  the  higher  the  field  strength,  the  lower  is  the  speed 
at  which  the  armature  consumes  the  voltage.  Since  the 
field  strength — neglecting  losses  and  saturation — is  propor- 
tional to  the  current,  the  speed  of  the  series  motor  would  be 
inversely  proportional  to  the  current,  as  shown  by  the  curve 
S0  in  Fig.  40. 

As  the  voltage  available  for  the  armature  rotation 
decreases  with  increasing  current,  from  e0  to  e,  by  the  ir 
drop  in  the  field  and  armature,  the  speed  decreases  in  the 
same  proportion,  from  the  curve  S0  to  the  curve  Si. 

In  reality,  however,  the  field  strength,  as  shown  by  the 
curve  M0,  is  proportional  to  the  current  only  at  low  cur- 
rents ;  but  for  higher  currents  the  field  strength  drops  below, 
by  magnetic  saturation,  as  shown  by  the  curve  M\  and 
ultimately,  at  very  high  currents,  it  becomes  nearly  con- 
stant. In  the  same  ratio  as  the  field  strength  drops  below 
proportionality  with  the  current,  the  speed  increases  and 
the  torque  decreases.  The  actual  speed  curve  is  therefore 
derived  from  the  curve  Si  by  increasing  the  values  of  the 
curve  Si  in  the  proportion,  M0  to  M,  and  is  given  by  the 
curve  S ;  and  in  the  same  proportion  the  torque  is  decreased 
to  the  curve  7\.  From  this  torque  curve  the  lost  torque  is 


ELECTRIC  RAILWAY:  MOTOR  CHARACTERISTICS    149 

now  subtracted;  that  is,  the  torque  representing  the  power 
consumed  in  friction  and  gear  losses,  hysteresis  and  eddy 
currents,  etc.  Some  of  the  losses  of  power  are  approxi- 
mately constant;  others  are  approximately  proportional  to 
the  square  of  the  current ;  and  the  lost  torque,  being  equal  to 
the  power  loss  divided  by  the  speed,  can  therefore  be  assumed 
as  approximately  constant:  somewhat  higher  at  low  and 
high  speeds,  as  shown  by  curve  F.  The  net  torque  then 
is  given  by  the  curve  T.  As  seen,  it  is  approximately  a 
straight  line,  passing  through  a  point  i0,  which  is  the  "run- 
ning light  current,"  and  its  corresponding  speed,  the  "free 
running  speed"  of  the  motor.  At  this  current  iot  the  speed 
is  highest ;  with  increase  of  current  it  drops  first  very  rap- 
idly, and  then  more  slowly;  and  the  higher  the  saturation 
of  the  motor  field  is,  the  slower  becomes  the  drop  of  speed 
at  high  currents. 

The  single-phase  alternating-current  motors  are  either 
directly  or  inductively  series  motors,  and  so  give  the  same 
general  characteristics  as  the  direct-current  series  motor. 
In  the  alternating-current  motors,  however,  in  addition  to 
the  ir  drop  an  ix  drop  exists;  that  is,  in  addition  to  the 
voltage  consumed  by  the  resistance,  still  further  voltage  is 
consumed  by  self-induction;  and  the  voltage  e  available 
for  the  armature  rotation  thus  drops  still  further,  as  seen  in 
Fig.  41.  Since  the  self-induction  consumes  voltage  in 
quadrature  with  the  current,  the  inductive  drop  is  not 
proportional  to  the  current,  but  is  small  at  low  currents, 
and  greater  at  high  currents;  e  therefore  is  not  a  straight 
line,  but  curves  downward  at  higher  currents.  The  speed, 
Si,  is  dropped  still  further  by  the  inductive  drop  of  voltage, 
to  the  curve  Si,  and  then  raised  to  the  curve  S  by  satura- 
tion. The  effect  of  saturation  in  the  alternating-current 
motor  usually  is  far  less,  since  the  magnetic  field  is  alter- 


150 


GENERAL  LECTURES 


nating,  and  good  power  factor  requires  a  low  field  excita- 
tion, and  therefore  high  saturation  cannot  well  be  reached. 


FlG.  41. — Alternating  current  series  motor  characteristic. 

The  torque  curves  are  the  same  as  in  the  direct-current 
motor,  except  that  the  effect  of  saturation  is  less  marked. 


ELECTRIC  RAILWA  Y:  MOTOR  CHARACTERISTICS    151 

In  efficiency,  the  shunt  or  induction  motor,  and  the 
series  motor  are  about  equal;  and  both  give  high  values  of 
efficiency  over  a  wide  range  of  current.  A  wide  range  of 
current,  however,  represents  a  wide  range  of  speed  in  the 
series  motor,  and  nearly  constant  speed  in  the  shunt 
motor;  therefore  while  the  series  motor  can  operate  at 
high  efficiency  over  a  wide  range  of  speed,  the  shunt  motor 
shows  high  efficiency  only  at  its  proper  speed. 

In  regard  to  the  effect  of  a  change  of  supply  voltage,  as  is 
caused,  for  instance,  by  a  drop  of  voltage  in  feeders  and 
mains,  the  series  motor  reacts  on  a  change  of  voltage  by  a 
corresponding  change  of  speed,  but  without  change  of 
current;  while  the  shunt  motor  and  induction  motor  react 
on  a  change  of  supply  voltage  by  a  change  of  current,  with 
little  or  no  change  of  speed.  As  the  limitation  of  a  system 
usually  is  the  current,  at  excessive  overloads  on  the  system, 
resulting  in  heavy  voltage  drop,  the  series  motors  run  slower, 
but  continue  to  move;  while  the  induction  motor  is  liable 
to  be  stalled. 


FOURTEENTH  LECTURE 
ALTERNATING-CURRENT  RAILWAY  MOTOR 

In  a  direct-current  motor,  whether  a  shunt  or  a  series 
motor,  the  motor  still  revolves  in  the  same  direction, 
if  the  impressed  e.m.f.  be  reversed,  as  field  and  arma- 
ture both  reverse.  Since  a  reversal  of  voltage  does  not 
change  the  operation  of  the  motor,  such  a  direct-current 
motor  therefore  can  operate  also  on  alternating  current. 
With  an  alternating  voltage  supply,  the  field  magnetism 
of  the  motor  also  alternates ;  the  motor  field  must  therefore 
be  laminated,  to  avoid  excessive  energy  losses  and  heating 
by  eddy  currents  (currents  produced  in  the  field  iron  by  the 
alternation  of  the  magnetism)  just  as  in  the  direct-current 
motor  the  armature  must  be  laminated. 

In  the  alternating-current  motor  in  addition  to  the  vol- 
tage consumed  by  the  resistance  of  the  motor  circuit  and 
that  consumed  by  the  armature  rotation,  voltage  is  also 
consumed  by  self-induction;  that  is,  by  the  alternation  of 
the  magnetism.  The  voltage  consumed  by  the  resistance 
represents  loss  of  power,  and  heating,  and  is  made  as 
small  as  possible  in  any  motor.  The  voltage  consumed  by 
the  rotation  of  the  armature,  or  " e.m.f.  of  rotation,"  is 
that  doing  the  useful  work  of  the  motor,  and  so  is  an  energy 
voltage,  or  voltage  in  phase  with  the  current;  just  as  the 
voltage  consumed  by  the  resistance  is  in  phase  with  the 
current.  The  voltage  consumed  by  self-induction,  due 
to  the  alternation  of  the  magnetism,  or  ''e.m.f.  of  alterna- 
tion," is  in  quadrature  with  the  current,  or  wattless;  that 
is,  it  consumes  no  power,  but  causes  the  current  to  lag, 

152 


ALTERNATING-CURRENT  RAILWAY  MOTOR       153 

and  so  lowers  the  power  factor  of  the  motor;  that  is,  causes 
the  motor  to  take  more  volt-amperes  than  corresponds 
to  its  output,  and  so  is  objectionable. 

The  useful  voltage,  or  e.m.f.  of  rotation  of  the  motor, 
is  proportional  to  the  speed;  or  rather  the  "frequency  of 
rotation,"  /„;  it  is  proportional  to  the  field  strength  F,  and 
to  the  number  of  armature  turns  m.  The  wattless  volt- 
age, or  self-induction  of  the  field,  is  proportional  to  the 
frequency  /,  to  the  field  strength  F,  and  the  number  of 
field  turns  n.  The  ratio  of  the  useful  voltage  to  the  watt- 
less voltage  therefore  is  m/0  -r-  «/,  and  to  make  the  useful 
voltage  high  and  the  wattless  voltage  low,  therefore  re- 
quires as  high  a  frequency  of  rotation  f0  and  as  low  a 
frequency  of  supply  /,  as  possible.  Thus  the  commutator 
motors  of  more  than  25  cycles  give  poor  power  factors; 
and  for  a  given  number  of  revolutions  /<,,  which  is  number 
of  revolutions  per  second  times  number  of  pairs  of  poles, 
therefore  is  the  higher,  the  more  poles  the  motor  has. 
Hence  a  greater  number  of  poles  are  generally  used  in  an 
alternating-current  than  in  a  direct-current  motor. 

Good  direct-current  motor  design  requires  a  strong  field 
and  weak  armature,  to  get  little  field  distortion  and  there- 
fore good  commutation;  that  is  high  n  and  low  m.  But 
such  proportions,  even  at  low  supply  frequency  /  and  high 
frequency  of  rotation  /0,  would  give  a  hopelessly  bad 
power  factor,  and  thus  a  commercially  impractical  motor. 
In  the  alternating-current  commutator  motor,  it  is  there- 
fore essential  to  use  as  strong  an  armature  and  as  weak  a 
field  (that  is,  as  large  a  number  of  armature  turns  m  and 
as  low  a  number  of  field  turns  n)  as  possible.  Very  soon, 
however,  a  limit  is  reached  in  this  direction,  even  if  the 
greater  field  distortion  and  the  resultant  bad  commutation 
were  not  to  be  considered:  the  armature  also  has  a  self- 


154  GENERAL  LECTURES 

induction ;  that  is,  the  alternating  magnetism  produced  by 
the  current  in  the  armature  turns  consumes  a  wattless 
e.m.f.  This  magnetism  is  small  in  a  direct-current  motor, 
but  with  many,  armature  turns  and  few  field  turns  it 
becomes  quite  considerable;  and  so,  while  a  further  de- 
crease of  the  field  turns  and  increase  of  the  armature  turns 
reduces  the  self-induction  of  the  field — which  varies  with 
the  square  of  the  field  turns — it  increases  the  self-induction 
of  the  armature — which  varies  with  the  square  of  the 
armature  turns.  There  is  thus  a  best  proportion  between 
armature  turns  and  field  turns,  which  gives  the  lowest 
total  self-induction.  This  is  about  in  the  proportion, 
armature  turns  m  to  field  turns  n  =  2  -f-  i ;  and  at  this 
proportion  the  power  factor  of  the  motor,  especially  at 
low  and  moderate  speeds,  is  still  very  poor. 

In  alternating-current  commutator  motors  it  is  therefore 
essential  to  apply  means  to  neutralize  the  armature  self- 
induction  and  armature  reaction,  so  as  to  be  able  to  in- 
crease the  proportion  of  armature  turns  to  field  turns 
sufficiently  to  get  good  power  factors.  This  is  done  by 
surrounding  the  armature  with  a  stationary  * '  compen- 
sating winding"  closely  adjacent  to  the  armature  con- 
ductors, located  in  the  field  pole  faces,  and  traversed  by  a 
current  opposite  in  direction  to  the  current  in  the  armature, 
and  of  the  same  number  of  ampere-turns;  so  that  the 
armature  ampere-turns  and  the  ampere-turns  of  the 
compensating  winding  neutralize  each  other,  and  the 
armature  reaction,  that  is,  the  magnetic  flux  produced  by 
the  armature  current,  and  the  self-induction  caused  by  it, 
disappear. 

This  compensating  winding  for  neutralizing  the  armature 
self-induction  was  introduced  by  R.  Eickemeyer  in  the  early 
days  of  the  alternating-current  commutator  motor,  and 


ALTERNATING-CURRENT  RAILWAY  MOTOR       155 

since  then  all  alternating-current  commutator  motors  have 
it;  so  that  the  electric  circuits  of  all  alternating-current 
commutator  motors  comprise  an  armature  winding  A,  a 
field  winding  F,  and  a  compensating  winding  C. 

Since  the  compensating  winding  cannot  be  identically  at 
the  same  place  as  the  armature  winding  (the  one  being 
located  in  slots  in  the  pole  faces,  the  other  in  slots  in  the 
armature  face)  there  still  exists  a  small  magnetic  flux 
produced  by  the  armature  winding:  the  "leakage  flux," 
analogous  to  the  leakage  flux  of  the  induction  motor;  and 
the  number  of  armature  turns  cannot  be  increased  in- 
definitely, otherwise  the  armature  self-induction,  due  to 
this  leakage  flux,  would  become  appreciable,  and  the 
power  factor  would  decrease  again.  The  minimum  total 
self-induction  of  the  motor  with  compensating  winding 
occurs  at  a  number  of  armature  turns  equal  to  three  to  five 
times  the  field  turns;  at  this  proportion,  the  power  factor 
is  already  very  good  at  low  speeds,  and  the  motor  is  in- 
dustrially satisfactory  in  this  regard. 

For  best  results,  that  is,  complete  compensation  and 
therefore  zero  magnetic  field  of  armature  reaction,  it  is, 
however,  necessary  not  only  to  have  the  same  number  of 
ampere-turns  in  the  compensating  winding  as  on  the 
armature,  but  also  to  have  these  ampere- turns  distributed 
in  the  same  manner  around  the  circumference.  With 
the  usual  armature  winding  this  is  not  the  case,  but  the 
armature  conductors  cover  the  whole  circumference; 
while  the  compensating  coil  conductors  cover  only  the 
pole  arc,  as  the  space  between  the  poles  is  taken  up  by  the 
field  winding.  That  is,  the  magnetic  distribution  around 
the  armature  circumference  is  as  shown  developed  in  Fig. 
42  :  the  field  gives  a  flat-topped  distribution,  the  armature  a 
peaked,  and  the  compensating  winding  has  a  small  flat  top 


156 


GENERAL  LECTURES 


ALTERNATING-CURRENT  RAILWAY  MOTOR       157 

and  with  the  total  ampere-turns  of  the  compensating  wind- 
ing equal  to  those  of  the  armature,  the  compensating 
winding  preponderates  in  front  of  the  field  poles,  the  arma- 
ture between  the  field  poles,  or  at  the  brushes,  and  there 
is  thus  a  small  magnetic  field  of  armature  reaction  remain- 
ing at  the  brushes,  just  where  it  is  objectionable  for 
commutation. 

As  it  is  not  feasible  to  distribute  the  compensating  wind- 
ing over  the  whole  circumference  of  the  stator,  the  armature 
winding  is  arranged  so  that  its  ampere-turns  cover  only  the 
pole  arcs.  This  is  done  by  using  fractional  pitch  in  the 
armature;  that  is,  the  spread  of  the  armature  coil  or  the 
space  between  its  two  conductors,  is  made,  not  equal  to  the 
pitch  of  the  pole,  as  shown  in  Fig.  43,  but  only  to  the  pitch 
of  the  pole  arcs,  as  shown  in  Fig.  44.  With  such  fractional 
pitch  winding,  the  currents  in  the  upper  and  the  lower  layer 
of  the  armature  conductors,  in  the  space  between  the  poles, 
flow  in  opposite  directions,  and  so  neutralize,  leaving 
only  that  part  of  the  armature  winding  in  front  of  the  pole 
arcs  as  magnetizing.  Hereby  the  distribution  of  the 
armature  ampere-turns  is  made  the  same  as  that  of  the 
compensating  winding,  and  so  complete  compensation  is 
realized. 

The  compensating  winding  may  be  energized  by  the  main 
current,  and  so  connected  in  series  with  the  field  and 
armature;  or  the  compensating  winding  may  be  short- 
circuited  upon  itself,  and  so  energized  by  an  induced 
current  acting  as  a  secondary  of  a  transformer  to  the 
armature  as  primary ;  and  as  in  a  transformer,  primary  and 
secondary  current  have  the  same  number  of  ampere- 
turns  (practically)  and  flow  in  opposite  directions,  such 
"inductive  compensation"  is  just  as  complete  compensa- 
tion as  the  "conductive  compensation"  produced  by 


158 


GENERAL  LECTURES 


passing    the    main    current    through    the    compensating 
winding. 

Vice  versa,   the  armature  may  be  short-circuited  and 
so  used  as  secondary  of  a  transformer,  with  the  compensat- 


FIG.  43. — Full  pitch  armature  winding. 

ing  winding  acting  as  primary.  In  either  of  these  motor 
types,  which  comprise  primary  and  secondary  circuits, 
that  is,  in  which  armature  and  compensating  winding  are 


FIG.  44. — Fractional  pitch  armature  winding. 

not  connected  directly  in  series,  but  inductively,  the  field 
may  be  energized  by  the  primary  or  supply  current,  or 
by  the  secondary  or  induced  current.  In  such  a  motor 


ALTERNATING-CURRENT  RAILWAY  MOTOR       159 

embodying  a  transformer  feature,  instead  of  impressing 
the  supply  voltage  upon  one  circuit  as  primary,  while  the 
other  is  closed  upon  itself  as  secondary,  the  supply  voltage 
may  be  divided  in  any  proportion  between  primary  and 
secondary. 

As  primary  and  secondary  current  of  a  transformer  are 
proportional  to  each  other,  it  is  immaterial,  regarding  the 
variation 'of  the  current  in  the  different  circuits  with  the 
load  and  speed,  whether  the  circuits  are  directly  in  series, 
or  by  transformation;  that  is,  all  these  motors  have  the 
same  speed — torque — current  characteristics,  as  discussed 
in  the  preceding  lecture,  and  differ  only  in  secondary 
effects,  mainly  regarding  commutation. 

The  use  of  the  transformer  feature  also  permits,  without 
change  of  supply  voltage,  to  get  the  effect  of  a  changed 
supply  voltage,  or  a  changed  number  of  field  turns,  by 
shifting  a  circuit  over  from  primary  to  secondary  or  vice 
versa.  For  instance,  if  the  armature  is  wound  with  half 
as  many  turns,  that  is,  for  half  the  voltage  and  twice  the 
current,  as  the  compensating  winding,  by  changing  the 
field  from  series  connection  with  the  compensating  wind- 
ing to  series  connection  with  the  armature,  the  current  in 
the  field  and  thus  the  field  strength,  is  doubled;  that  is 
the  same  effect  is  produced  as  would  be  by  doubling  the 
number  of  field  turns. 

According  to  the  relative  connection  of  the  three  circuits, 
armature  A,  compensating  circuit  C,  and  field  F,  alter- 
nating-current commutator  motors  of  the  series  type  can 
be  divided  into  the  classes  shown  diagramatically  in 
Fig.  45 : 
Primary :  Secondary : 

A  +  C  +  F  Conductively    Compensated 

Series  Motor  (2). 


160 


GENERAL  LECTURES 


ALTERNATING-CURRENT  RAILWAY  MOTOR       161 

A  +  F  C  Inductively  Compensating 

Series  Motor  (3). 

A  C  +  F  Inductively  Compensating 

Series  Motor  with  Second- 
ary Excitation,  or  In- 
verted Repulsion  Motor 

(4). 

C  +  F  A  Repulsion  Motor  (5). 

C  A  +  F  Repulsion     Motor     with 

Secondary  Excitation  (6). 
C  &  A  +  F  Series    Repulsion    Motor    A 

(7). 

C  -h  F  &  A  Series    Repulsion    Motor    B 

(8). 

The  main  difference  between  these  types  of  motors  is 
found  in  their  commutation. 

In  a  direct-current  motor,  with  the  brushes  set  at  the 
neutral,  that  is,  midway  between  the  field  poles  (as  is 
customary  in  a  reversible  motor  like  the  series  motor),  the 
armature  turn,  which  is  short-circuited  under  the  brush 
during  the  commutation,  encloses  all  the  lines  of  magnetic 
force  of  the  field;  so  during  this  moment  it  does  not  cut 
any  lines  of  force  by  its  rotation,  and  thus  no  e.m.f.  is 
induced  in  this  turn;  that  is,  no  current  is  produced,  if 
the  armature  reaction  is  compensated  for,  or  is  otherwise 
negligible.  If  the  motor  has  a  considerable  armature  re- 
action, and  thus  a  magnetic  field  at  the  brushes,  this 
magnetic  field  of  armature  reaction  induces  an  e.m.f.  in 
the  short-circuited  turn  under  the  brush,  and  so  causes 
sparking.  Hence  high  armature  reaction  impairs  the 
commutation  of  the  motor. 

In   an   alternating-current   series   motor   the   armature 
11 


162  GENERAL  LECTURES 

reaction  is  neutralized  by  the  compensating  winding,  and 
therefore  no  magnetic  field  of  armature  reaction  exists; 
hence  no  e.m.f .  is  induced  in  the  turn  short-circuited  under 
the  brush  by  its  rotation  through  the  magnetic  field.  As 
this  field,  however,  is  alternating,  an  e.m.f.  is  induced  in 
the  short-circuited  turn  by  the  alternations  of  the  lines  of 
magnetic  force  enclosed  by  it,  and  causes  a  short-circuit 
current  and  in  that  way,  sparking.  This  e.m.f.,  being 
due  to  the  alternation  of  the  enclosed  field  flux,  is  inde- 
pendent of  the  speed  of  rotation;  it  also  exists  with  the 
motor  at  a  standstill,  and  is  a  maximum  in  the  armature 
turn  under  the  brush,  as  this  encloses  the  total  field  flux. 
The  position  of  the  armature  turn  during  commutation, 
which  in  a  direct-current  motor  is  the  position  of  zero 
induced  e.m.f.,  is  therefore  in  an  alternating-current  motor, 
the  position  of  maximum  induced  e.m.f.,  but  induced  not 
by  the  rotation  of  the  turn,  but  by  the  alternation  of  the 
magnetism.  That  is,  this  turn  is  in  the  position  of  a 
short-circuited  secondary  to  the  field  coil  of  the  motor 
as  primary  of  a  transformer;  and  as  primary  and  second- 
ary ampere-turns  in  a  transformer  are  approximately 
equal,  the  current  in  the  armature  turn  during  commutation 
is  very  large ;  if  not  limited  by  the  resistance  or  reactance  of 
the  coil,  it  is  as  many  times  greater  than  the  full-load 
current,  as  the  field  coil  has  turns.  This  causes  serious 
sparking,  if  not  taken  care  of. 

One  way  of  mitigating  the  effect  of  this  short-circuit  cur- 
rent is  to  reduce  it  by  interposing  resistance  or  reactance; 
that  is  by  making  the  leads  between  the  armature  turns 
and  the  commutator  bars  of  high  resistance  or  high  re- 
actance. Obviously  this  arrangement  can  merely  some- 
what reduce  the  sparking  by  reducing  the  current  in  the 
short-circuited  coil,  but  can  not  eliminate  it ;  and  it  has  the 


ALTERNATING-CURRENT  RAILWAY  MOTOR       163 

disadvantage,  that  in  the  moment  of  starting,  if  the  motor 
does  not  start  at  once,  the  resistance  lead  is  liable  to  be 
burned  out  by  excessive  heating;  while  when  running, 
each  lead  is  in  circuit  only  a  very  small  part  of  the  time: 
during  the  moment  when  the  armature  turn  to  which 
it  connects,  is  under  a  commutator  brush.  As  the  re- 
sistance of  the  lead  must  be  very  much  greater  than  that 
of  the  armature  coil,  and  as  the  space  available  for  it  is 
very  much  smaller,  if  remaining  in  circuit  for  any  length 
of  time  it  is  destroyed  by  heat. 

In  direct-current  motors,  commutation  may  be  controlled 
by  an  interpole  or  commutating  pole ;  that  is,  by  producing  a 
magnetic  field  at  the  brush,  in  direction  opposite  to  the  field 
of  armature  reaction,  and  by  this  field  inducing  in  the  arma- 
ture turn  during  commutation,  an  e.m.f.  of  rotation  which 
reverses  the  current.  Such  commutating,  poles  are  becom- 
ing extensively  used  in  larger  direct-current  railway 
motors.  Such  a  commutating  pole,  connected  in  series 
into  a  circuit,  would,  in  the  alternating-current  motor,  in- 
induce  an  e.m.f.  in  the  short-circuited  turn,  by  its  rotation; 
but  this  e.m.f.  would  be  in  phase  with  the  field  of  the  com- 
mutating pole,  and  thus  with  the  current,  that  is,  with  the 
main  field  of  the  motor.  Therefore  it  could  not  neutralize 
the  e.m.f.  induced  in  the  short-circuited  turn  by  the  alter- 
nation of  the  main  field  through  it,  since  this  latter  e.m.f. 
is  in  quadrature  with  the  main  field,  and  thus  with  the  cur- 
rent; but  would  simply  add  itself  to  it,  and  so  make  the 
sparking  worse.  A  series  commutating  pole,  while  effective 
in  a  direct-current  motor,  is  therefore  ineffective  in  an  alter- 
nating-current motor,  due  to  its  wrong  phase. 

To  neutralize  the  e.m.f.  induced  by  the  alternation  of 
the  main  field  through  the  armature  turn  during  com- 
mutation, by  an  opposite  e.m.f.  induced  in  this  turn  by  its 


164  GENERAL  LECTURES 

rotation  through  a  quadrature  field  or  commut-ating  field, 
this  field  must  therefore  have  the  proper  phase.  The  e.m.f. 
of  alternation  of  the  main  field  through  the  short-circuited 
turn  is  proportional  to  the  main  field  F  and  frequency 
/,  and  is  in  quadrature  with  the  main  field.  The  e.m.f. 
induced  in  the  short-circuited  turn  by  its  rotation  through 
the  commutating  field  is  proportional  to  the  frequency  of 
rotation  or  speed  /„,  and  to  the  commutating  field  F0, 
and  in  phase  therewith;  to  be  in  opposition  and  equal  to 
the  e.m.f.  of  alternation,  the  commutating  field  must 
therefore  be  in  quadrature  with  the  main  field,  and  fre- 
quency times  main  field  must  equal  speed  times  commutat- 
ing field.  That  is : 

fF  =  f,F0 
or  in  other  words,  the  commutating  field  must  be : 

F.  =  /F 

Jo 

or  equal  to  the  main  field  times  the  ratio  of  frequency  to 
speed,  and  in  quadrature  therewith. 

Hence,  at  synchronism  :/„=/,  the  commutating  field  must 
be  equal  to  the  main  field;  at  half  synchronism : /0  =  J^/,  it 
must  be  twice;  at  double  synchronism:  /„  =  2/,  it  must  be 
one-half  the  main  field. 

The  problem  of  controlling  the  commutation  of  the  alter- 
nating-current motor  therefore  requires  the  production  of  a 
commutating  field  of  proper  strength,  in  quadrature  phase 
with  the  main  field  of  the  motor,  and  thus  with  the  current. 

In  a  transformer,  on  non-inductive  or  nearly  non-induct- 
ive secondary  load,  the  magnetism  is  approximately  in 
quadrature  behind  the  primary,  and  ahead  of  the  secondary 
current ;  transformation  between  compensating  winding  and 
armature  thus  offers  a  means  of  producing  a  quadrature 
field  in  the  alternating-current  motor  for  compensation. 


ALTERNATING-CURRENT  RAILWAY  MOTOR       165 

In  the  conductively  compensated  series  motor,  at  perfect 
compensation,  no  quadrature  field  exists;  while  with  over 
or  undercompensation,  a  quadrature  field  exists,  in  phase 
with  the  current,  and  therefore  not  effective  as  commutat- 
ing  field. 

In  the'  inductively  compensated  series  motor,  the  quad- 
rature field,  which  transforms  current  from  the  armature  to 
the  compensating  winding,  is  of  negligible  intensity,  as  the 
compensating  winding  is  short-circuited,  and  thus  con- 
sumes very  little  voltage. 

A  quadrature  field,  however,  appears  in  those  motors  in 
which  the  compensating  winding  is  primary,  and  the 
armature  secondary,  that  is  in  repulsion  motors;  since  in 
the  armature  the  induced  or  transformer  e.m.f.  is  opposed 
by  the  e.m.f.  of  rotation;  so  a  considerable  e.m.f.  is  in- 
duced, and  therefore  a  considerable  transformer  flux 
exists. 

Therefore,  when  impressing  the  supply  voltage  on  the 
compensating  winding,  and  short-circuiting  the  armature 
upon  itself,  that  is,  in  the  repulsion  motor,  the  voltage  is 
supplied  to  the  armature  by  transformation  from  the 
compensating  winding,  and  the  magnetic  flux  of  this  trans- 
former is  in  quadrature  with  the  supply  current;  that  is, 
it  has  the  proper  phase  as  commutating  flux. 

The  repulsion  motor  thus  has  in  addition  to  the  main 
field,  in  phase  with  the  current,  a  transformer  field,  in 
quadrature  with  the  main  field  in  space  and  in  time,  and  so 
in  the  proper  direction  and  phase  as  commutating  field; 
thus  giving  perfect  commutation  if  this  transformer  field 
has  the  intensity  required  for  commutation,  as  discussed 
above. 

As  in  the  repulsion  motor,  the  armature  is  short-circuited 
upon  itself,  the  voltage  supplied  to  it  by  transformation 


166  GENERAL  LECTURES 

from  the  compensating  winding  equals  the  voltage  con- 
sumed in  it  by  the  rotation  through  the  main  field.  The 
former  voltage  is  proportional  to  the  frequency  /  and  to 
the  transformer  field  F1,  the  latter  to  the  speed  f0  and  to 
the  main  field  F,  and  it  so  is  : 


that  is,  the  transformer  field  is  : 

Fi  =  tjF 

or  equal  to  the  main  field   times   the  ratio   of   speed  to 
frequency. 

Comparing  this  value  of  the  transformer  field  of  the  re- 
pulsion motor,  F1,  with  the  required  commutating  field  F0, 
it  is  seen  that  at  synchronism  f0  =  /,  F1  =  F0;  that  is,  the 
transformer  field  of  the  repulsion  motor  has  the  proper  value 
as  commutating  field,  so  that  no  short-circuit  current  is 
produced  in  the  armature  turn  under  the  brush,  but  the 
commutation  is  as  good  as  in  a  direct-current  motor  with 
negligible  armature  reaction. 

At  half  synchronism,  /„  =  >£/,  the  transformer  field  of 
the  repulsion  motor:  F1  =  J^F,  is  only  one-quarter  as  large 
as  the  commutating  field  required  F0  =  2F,  and  the  short- 
circuit  current  is  reduced  by  25  per  cent,  below  the  value 
which  it  has  in  the  series  motor;  and  the  commutation,  while 
it  is  better,  is  not  yet  perfect. 

At  double  synchronism:  /„  =  2/,  the  transformer  field 
is  F1  =  2F,  while  the  commutation  field  should  be  :  F0  = 
%F,  and  the  transformer  field  thus  is  four  .  times  larger 
than  it  should  be  for  commutation  ;  so  that  only  one-quarter 
of  the  transformer  field  is  used  to  neutralize  the  e.m.f.  of 
alternation  in  the  short-circuited  turn;  the  other  three- 
quarters  induces  an  e.m.f.,  thus  causing  a  short-circuit  cur- 


ALTERNATING-CURRENT  RAILWAY  MOTOR       167 

rent  three  times  as  large  as  it  would  be  in  a  series  motor. 
That  is,  the  short-circuit  current  under  the  brush,  and  thus 
the  sparking,  in  the  repulsion  motor  at  double  synchronism 
is  very  much  worse  than  in  the  series  motor,  and  the  repul- 
sion motor  at  these  high  speeds  is  practically  inoperative. 

Hence,  as  regards  commutation,  a  repulsion  motor  is 
equal  to  the  series  motor  at  standstill  where  no  compensa- 
tion of  the  short-circuit  current  is  possible — but  becomes 
better  with  increasing  speed :  as  good  as  a  good  direct-cur- 
rent series  motor  at  synchronism;  and  then  again  becomes 
worse  by  overcompensation,  until  at  some  speed  above 
synchronism,  it  again  becomes  as  poor  as  the  alternating- 
current  series  motor;  above  this  speed,  it  becomes  rapidly 
inferior  to  the  series  motor. 

To  produce  right  intensity  of  the  transformer  field,  to  act 
as  commutating  field,  it  is  therefore  necessary  above  syn- 
chronism to  reduce  the  transformer  field  below  the  value 
which  it  would  have  when  transforming  the  total  supply 
voltage  from  compensating  winding  to  armature.  This 
means,  that  above  synchronism,  only  a  part  of  the  supply 
voltage  must  be  transformed  from  compensating  winding 
to  armature,  the  rest  directly  impressed  upon  the  armature. 
Thus  at  double  synchronism,  where  the  transformer  field 
of  the  repulsion  motor  is  four  times  as  strong  as  is  required 
for  commutation,  to  reduce  it  to  one-quarter,  only  one- 
quarter  of  the  supply  voltage  must  be  impressed  upon  the 
compensating  winding,  three-quarters  directly  on  the 
armature. 

To  get  zero  short-circuit  current  in  the  armature  turn 
under  the  brush,  below  synchronism  more  than  the  full 
supply  voltage  would  have  to  be  impressed  upon  the  com- 
pensating winding,  which  usually  cannot  conveniently 
be  done.  At  synchronism  the  full  supply  voltage  is  im- 


168  GENERAL  LECTURES 

pressed  upon  the  compensating  winding,  while  the  armature 
is  short-circuited  as  repulsion  motor;  and  with  increasing 
speed  above  synchronism,  more  and  more  of  the  supply 
voltage  is  shifted  over  from  compensating  winding  to  arma- 
ture ;  that  is,  the  voltage  impressed  upon  the  compensating 
winding  is  reduced,  from  full  voltage  at  synchronism,  while 
the  voltage  impressed  upon  the  armature  is  increased,  from 
zero  at  synchronism,  to  about  three-quarters  of  the  supply 
voltage  at  double  synchronism.  Such  a  motor,  in  which  the 
transformer  field  is  varied  in  accordance  with  the  require- 
ment of  commutation,  is  called  a  "series  repulsion  motor." 
The  arrangement  described  here  eliminates  the  short-cir- 
cuit current  induced  in  the  commutated  armature  turn  by 
the  alternation  of  the  main  field,  and  that  completely  above 
synchronism,  so  that  during  commutation,  no  current  is 
induced  in  the  armature  turn.  This,  however,  is  not  suffi- 
cient for  perfect  commutation:  during  the  passage  of  the 
armature  turn  under  the  brush,  the  current  in  the  turn 
should  reverse;  so  that  in  the  moment  in  which  the  turn 
leaves  the  brush,  the  current  has  already  reversed.  For 
sparkless  commutation,  it  therefore  is  necessary,  in  addi- 
tion to  the  neutralizing  e.m.f.  of  the  transformer  field,  to 
induce  an  e.m.f.  which  reverses  the  current.  This  e.m.f., 
and  thus  the  magnetic  flux  which  induces  it  by  the  rotation, 
must  be  in  phase  with  the  current.  That  is,  in  addition 
to  the  "neutralizing"  component  of  the  commutating 
field  (which  is  in  quadrature  with  the  current),  to  reverse 
the  current,  a  second  component  of  the  commutating  field 
must  exist,  in  phase  with  the  current;  this  component  so 
may  be  called  the  "reversing  field."  The  total  commutat- 
ing field  required  to  eliminate  the  short-circuit  current  due 
to  the  alternating  main  field  by  the  "neutralizing"  flux, 
and  to  reverse  the  armature  current  by  the  "reversing 


ALTERNATING-CURRENT  RAILWAY  MOTOR       169 

flux,"  must  therefore  be  somewhat  less  than  90°  lagging 
behind  the  main  field  and  thus  the  main  current. 

While  in  a  transformer  with  non-inductive  load  on  the 
secondary,  the  magnetic  flux  lags  nearly  90°  behind  the 
primary  current,  in  a  transformer  with  inductive  load  on  the 
secondary,  the  magnetic  flux  lags  less  than  90°  behind  the 
primary  current ;  and  the  more  so  the  higher  the  inductivity 
of  the  secondary  load. 

Therefore,  by  putting  a  reactance  into  the  armature 
circuit  of  the  motor,  and  so  making  the  armature  circuit 
inductive,  the  transformer  flux  is  made  to  lag  less  than  90° 
behind  the  current,  and  act  not  only  as  neutralizing  but 
also  as  reversing  flux;  and  so,  if  it  be  of  proper  intensity,  it 
gives  perfect  commutation. 

An  additional  reactance  would  in  general  be  objectional, 
in  lowering  the  power  factor  of  the  motor.  The  motor, 
however,  contains  a  reactance:  its  field  circuit,  which  has  to 
be  excited,  can  be  used  as  reactance  for  the  armature  cir- 
cuit. That  is,  by  connecting  the  field  coils  into  the  arma- 
ture circuit,  or  in  other  words,  using  secondary  excitation, 
the  transformer  flux  of  the  motor  is  given  the  lead  ahead  of 
quadrature  position  with  the  main  field,  which  is  required 
to  act  as  reversing  field.  However,  hereby  the  power 
factor  of  the  motor  is  somewhat  lowered. 

In  this  manner,  it  is  possible  in  the  alternating-current 
commutator  motor,  to  get  at  all  speeds  from  synchronism 
upwards,  the  same  perfect  commutation  as  in  a  direct-cur- 
rent motor  with  commutating  poles,  by  varying  the  distri- 
bution of  supply  voltage  between  compensating  winding 
and  armature,  and  exciting  the  field  in  series  with  the  arma- 
ture circuit ;  that  is,  in  the  series  repulsion  motor  B  of  the 
preceding  table.  Obviously,  this  distribution  of  voltage 
would  for  all  practical  purposes  be  carried  out  sufficiently 


170 


GENERAL  LECTURES 
T 


FIG.  46. 


ALTERNATING-CURRENT  RAILWAY  MOTOR       171 

by  using  a  number  of  steps,  as  shown  diagrammatically  by 
the  arrangement  in  Fig.  46 : 

T  is  the  supply  circuit,  F  the  field  winding,  A  the  arma- 
ture, and  C  the  compensating  winding. 

Closing  switch  i,  and  leaving  all  others  open,  the  motor 
is  a  repulsion  motor. 

Closing  switch  2,  and  leaving  i  and  all  others  open,  the 
motor  is  a  repulsion  motor  with  secondary  excitation. 

Closing  switch  3,  or  4,  5  .  .  .  and  leaving  all  others 
open,  the  motor  is  a  series  repulsion  motor  B,  with  gradually 
increasing  armature  voltage  and  decreasing  voltage  on  the 
compensating  winding. 

By  winding  the  armature  for  half  the  voltage  and  twice 
the  current  of  the  compensating  winding,  when  changing 
from  position  i,  the  field  in  the  compensating  circuit,  to  the 
next  position,  with  the  field  in  the  armature  circuit,  the 
field  current  and  the  field  strength  becomes  double  the 
value  it  had  in  starting,  where  no  compensation  exists, 
and  which  it  would  have  to  maintain  in  a  series  motor ;  and 
thus  a  correspondingly  greater  motor  output  is  secured, 
than  would  be  possible  in  a  motor  in  which  the  commuta- 
tion is  not  controlled. 


FIFTEENTH  LECTURE 
ELECTROCHEMISTRY 

Electrochemistry  is  one  of  the  most  important  applica- 
tions of  electric  power,  and  possibly  even  more  power  is 
used  for  electrochemical  work  than  for  railroading. 

In  electrochemical  industries  the  most  expensive  part  is 
electric  power;  material  and  labor  are  usually  much  less. 
Such  industries  therefore  are  located  at  water  powers, 
where  the  cost  of  power  is  very  low. 

The  main  classes  of  electrochemical  work  are: 

A.  Electrolytic. 

B.  Electrometallurgical. 

A.  ELECTROLYTIC  WORK 

The  chemical  action  of  the  current  is  used,  by  electro- 
lyzing  either  solutions  of  salts  or  fused  salts  or  compounds. 

Electrolysis  of  solutions  in  water  is  possible  only  with 
such  metals  which  have  less  chemical  affinity  than  hy- 
drogen. For  instance,  Cu,  Fe,  and  Zn  can  be  deposited 
from  salt  solutions '  in  water,  but  not  Al,  Mg,  Na,  etc. 
Electrolyzing,  for  instance,  NaCl  (salt  solution)  the 
sodium  (Na)  which  appears  at  the  negative  terminal 
immediately  dissociates  the  water  and  gives  Na  +  H2O  = 
NaOH  +  H,  or:  sodium  plus  water  =  caustic  soda  plus 
hydrogen. 

It  takes  1.4  volts  to  electrolyze  water;  any  metal  re- 
quiring more  than  1.4  volts  for  separation  therefore  is  not 

separated,  but  hydrogen  is  produced. 

172 


ELECTROCHEMISTRY  173 

Therefore  the  highest  voltage  used  in  an  electrolytic  cell 
containing  water  is  1.4  +  the  ir  drop  in  the  resistance  of  the 
cell ;  which  latter,  for  reasons  of  economy,  is  made  as  low  as 
possible. 

Even  fused  salts  require  fairly  low  voltage,  at  the  highest 
from  3  to  4  volts. 

Since  the  voltage  required  per  cell  is  very  low,  a  large 
number  of  cells  are  connected  in  series,  and  even  then  large 
low- volt  age  machines  are  required. 

Some  of  the  important  applications  of  electrolysis  are : 

Electroplating;  that  is,  covering  with  copper,  nickel, 
silver,  gold,  etc. 

Electrotyping;  that  is,  making  of  copies,  usually  of  cop- 
per; and  especially 

Metal  refining. 

A  very  large  part  of  all  the  copper  used  is  electrically 
refined.  The  crude  copper  as  cast  plate  is  used  as  anode  or 
positive,  and  a  thin  plate  of  refined  copper  is  used  as 
cathode,  or  negative  terminal  in  a  copper  sulphate  solution. 
The  anode  is  dissolved  by  the  current  and  the  fine  copper 
is  deposited  on  the  cathode;  while  silver  and  gold  go  down 
into  the  mud,  lead  goes  into  the  mud  as  sulphate,  tin  as 
oxide;  sulphur,  selenium  and  tellurium,  arsenic  and  other 
impurities  also  go  in  the  mud;  and  zinc  and  iron  remain  in 
solution  as  sulphates  if  the  current  density  is  sufficiently 
low.  If  the  current  density  is  high,  some  zinc  and  iron 
may  deposit :  zinc  and  iron  have  a  greater  chemical  activity 
than  copper,  since  they  precipitate  copper  from  solution. 
Therefore,  it  takes  more  power,  that  is,  more  voltage, 
to  deposit  zinc  and  iron,  than  it  takes  to  deposit  copper. 
If  the  current  density  is  low,  the  voltage  required  to  deposit 
the  copper  plus  the  ir  drop,  that  is,  the  total  voltage  of 
the  cell,  is  less  than  the  voltage  required  to  deposit  zinc  or 


174  GENERAL  LECTURES 

iron,  and  they  do  not  deposit,  but  dissolve  at  the  anode  and 
remain  in  solution. 

At  higher  current  density  the  ir  drop  in  the  cell  is  higher ; 
thus  the  total  voltage  of  the  cell  is  higher,  and  may  become 
high  enough  to  deposit  iron  or  even  zinc. 

If  the  anode  is  crude  copper,  the  cathode  pure  copper, 
the  voltage  at  the  anode  is  higher  than  at  the  cathode  and 
the  cell  takes  some  voltage.  The  voltage  required  for 
copper  refining  is  the  higher,  the  more  impure  the  copper 
is;  but  is  always  very  low,  usually  a  fraction  of  a  volt, 
and  therefore  very  many  cells  are  run  in  series. 

The  solution  gradually  becomes  impure  and  has  to  be 
replaced. 

Other  metals  as  zinc,  lead,  iron,  etc.,  are  occasionally 
refined  electrolytically,  but  not  to  the  same  extent  as 
copper. 

Metal  Reduction. — Metals  are  reduced  from  their  ores 
electrolytically,  especially  such  metals  which  have  so  high 
chemical  affinity  that  they  are  not  reduced  by  heating  with 
carbon.  In  this  way  aluminum,  magnesium,  sodium, 
calcium,  etc.,  are  made  electrolytically.  Since  their  chemi- 
cal affinity  is  greater  than  that  of  hydrogen,  they  cannot 
be  deposited  from  solutions  in  water,  but  only  from  fused 
salts,  or  solutions  in  fused  salts.  So  calcium  is  produced 
now  by  electrolyzing  fused  calcium  chloride,  CaCl2. 
Aluminum  is  made  by  electrolyzing  a  solution  of  alumina 
in  melted  cryolite  (sodium  aluminum  fluoride). 

Secondary  Products. — Frequently  electrolysis  is  used 
to  produce  not  the  substances  which  are  directly  deposited, 
but  substances  produced  by  the  reaction  of  these  deposits 
on  the  solutions.  For  instance,  electrolyzing  a  solution 
of  salt,  NaCl,  in  water,  we  get  sodium,  Na,  at  the  negative, 
chlorine,  Cl,  at  the  positive  terminal. 


ELECTROCHEMISTRY  175 

If  we  use  mercury,  Hg,  as  negative  electrode,  it  dissolves 
the  sodium  and  so  we  get  sodium  amalgam. 

Otherwise  the  sodium  does  not  deposit  but  immediately 
acts  upon  the  water  and  forms  sodium  hydrate  or  caustic 
soda,  NaOH. 

The  chlorine,  Cl,  at  the  anode  also  reacts  on  the  water, 
one  chlorine  atom  taking  up  one  hydrogen  and  another 
chlorine  atom  the  remaining  OH  of  the  water,  H^O; 
that  is,  we  get  2C1  +  H2O  =  C1H  +  C1OH,  that  is, 
hydrochloric  +  hypochlorous  acid. 

With  the  sodium  hydrate  from  the  other  cathode  these 
acids  form  NaCl  and  ClONa,  that  is  sodium  chloride  and 
hypochlorite,  or  bleaching  soda. 

If  the  solution  is  hot,  the  reaction  goes  further  and  we  get 
6C1  +  3H2O  =  sQH  +  C1O3H,  that  is  hydrochloric  and 
chloric  acid,  and  with  the  sodium  hydrate  from  the  other 
side  these  form  NaCl  and  ClO3Na,  that  is,  sodium  chloride 
and  sodium  chlorate. 

In  this  way  considerable  industries  have  developed,  pro- 
ducing electrolytically  caustic  soda,  bleaching  soda,  and 
chlorates. 

Alternating  current  is  used  very  little  for  electrolytic 
work,  as  with  organic  compounds  to  produce  oxidation  and 
reduction  at  the  same  time;  that  is,  act  on  the  compound  in 
rapid  succession  by  oxygen  and  hydrogen,  the  one  during 
the  one,  the  other  during  the  next  half  wave  of  current. 

Very  active  metals  like  manganese  and  silicon  dissolve  by 
alternating  current ;  that  is,  one-half  wave  dissolves,  but  the 
other  does  not  deposit  again. 

Very  inert  metals  like  platinum  are  deposited  by  alterna- 
ting current;  that  is,  the  negative  half  wave  deposits  by 
alternating  current,  but  the  positive  half  wave  does  not 
dissolve. 


176  GENERAL  LECTURES 

B.  ELECTROMETALLURGICAL  WORK 

In  electrometallurgical  work  the  heat  is  used  to  produce 
the  chemical  action;  thus  it  is  immaterial  whether  alter- 
nating or  direct  current  is  used. 

The  voltage  required  is  still  low  but  not  as  low  as  in 
electrolytic  work: 

The  carborundum  furnace  takes  from  250  to  90,  mostly 
about  100  volts;  that  is,  it  starts  cold  with  250  volts. 
While  heating  up  the  resistance  drops,  and  the  voltage 
decreases  down  to  100  volts  when  the  furnace  is  hot  and 
remains  there  until  toward  the  end.  Then  the  inner 
layer  of  carborundum  begins  to  change  to  graphite  and 
the  resistance,  and  therefore  the  voltage  falls. 

The  carbide  furnace  and  arc  furnaces  in  general  take 
from  50  to  100  volts;  the  graphite  furnace  takes  from  10 
to  20  volts. 

To  get  very  high  temperatures  a  very  large  amount  of 
energy  has  to  be  concentrated  in  one  furnace ;  and  with  the 
moderate  voltage  used,  this  requires  very  large  currents, 
thousands  of  amperes.  Alternating  currents  are  almost 
exclusively  used,  since  it  is  easier  to  produce  very  large 
alternating  currents  by  transformers,  and  since  it  is  easier 
to  control  alternating  than  direct  currents. 

Electric  heat  necessarily  is  very  much  more  expensive 
than  heat  produced  by  burning  coal,  and  so  the  electric 
furnace  is  used  mainly : 

First. — Where  very  perfect  control  of  the  temperatures 
and  freedom  from  impurities  is  essential. 

Second. — Where  temperatures  higher  than  can  be  pro- 
duced by  combustion  are  required. 

i.  Very  accurate  temperature  regulation  and  freedom 
from  impurities,  for  instance,  are  important  in  making  and 
annealing  high-grade  tool  steels,  etc.  By  using  coal  or  oil 


ELECTROCHEMISTRY  177 

as  fuel,  contamination  by  the  gases  of  combustion,  and  by 
the  metal  taking  up  carbon  (or  if  an  excess  of  air  is  used, 
oxygen)  is  difficult  to  avoid. 

By  electric  heating,  by  resistance  at  lower  temperature  and 
by  induction  furnace  at  higher  temperature,  contamination 
can  be  perfectly  avoided  and  even  the  air  can  be  excluded. 

2.  The  temperature  of  combustion  is  limited. 

Four-fifths  of  air  is  nitrogen  which  does  not  take  part  in 
the  combustion,  but  which  has  to  be  heated,  thus  greatly 
lowering  the  temperature;  therefore  combustion  in  air, 
even  if  the  air  is  preheated,  gives  a  lower  temperature  than 
when  using  oxygen.  But  even  the  temperature  of  the 
oxy-hydrogen,  or  the  oxy-acetylene  flame  is  only  just 
able  to  melt  platinum. 

The  temperature  which  can  be  reached  by  combustion,  is 
limited,  since  at  very  high  temperature  the  chemical  affinity 
of  oxygen  for  hydrogen  and  carbon  ceases :  water  dissociates, 
that  is,  spontaneously  splits  up  in  hydrogen  and  oxygen  at 
2ooo°C.  and  no  temperature  higher  than  2000°  can  there- 
fore be  reached  by  the  oxy-hydrogen  flame ;  carbon  dioxide, 
CO2,  already  dissociates  at  about  i5oo°C.  into  carbon 
monoxide,  CO,  and  oxygen,  O.  Carbon  monoxide,  CO, 
splits  up  into  carbon  and  oxygen  not  much  above  2ooo°C. 
(In  all  high  temperature  reactions  of  carbon,  as  in  the 
formation  of  carbides,  CO  therefore  always  forms  and 
not  CO2,  since  CO2  cannot  exist  at  a  very  high  temperature; 
and  the  CO  when  leaving  the  furnace  then  burns  to  CO2 
with  blue  flame.) 

Higher  temperatures  than  those  generated  by  the  com- 
bustion of  carbon  and  hydrogen  can  be  produced  by  the 
combustion  of  those  elements  whose  oxides  are  stable  at 
very  high  temperatures,  as  aluminum  and  calcium.  In 
this  way,  many  metals,  as  chromium  and  manganese,  which 

12 


178  GENERAL  LECTURES 

cannot  be  reduced  from  the  oxides  by  carbon  (due  to  the 
lower  temperature  of  carbon  combustion)  can  be  reduced  by 
aluminum  in  the  "thermite"  process.  That  is,  their  oxides 
are  mixed  with  powdered  aluminum  and  then  ignited :  the 
aluminum  burns  in  taking  up  the  oxygen  of  the  metal,  and 
so  produces  an  extremely  high  temperature,  which  melts 
the  metal  and  the  alumina  (corundum)  which  is  produced. 

Since,  however,  all  the  aluminum  is  made  electrolytically, 
the  thermite  process  still  requires  the  use  of  electric  power. 
The  temperature  of  combustion  of  aluminum,  however,  is 
still  far  below  that  of  the  electric  carbon  arc,  since  in  the 
carbon  arc,  alumina  boils. 

For  temperatures  above  2000°  to  25oo°C.,  and  up  to  the 
arc  temperature  or  about  35oo°C.,  electric  energy  is  there- 
fore necessary. 

Electric  furnaces  are  of  two  classes: 

Arc  Furnaces  and  Resistance  Furnaces. 

In  the  -  resistance  furnace  any  temperature  can  be  pro- 
duced up  to  the  point  of  destruction  of  the  resistance  mate- 
rial, that  is,  up  to  35oo°C.,  when  using  carbon. 

The  arc  furnace  gives  the  arc  temperature  of  35oo°C., 
but  allows  the  concentration  of  much  more  energy  in  a  small 
space  and  thus  produces  reactions  requiring  the  very  highest 
temperatures. 

Some  of  the  electrometallurgical  industries  are : 

(a)  Calcium    carbide    production.     Arc    furnaces    are 
used  and  the  reaction  is 

CaO  +  sC  =  CaC2  +  CO. 

A  mixture  of  coke  and  quick  lime  is  used  in  the  process. 

(b)  Carborundum  production.     A  resistance  furnace  is 
used,  containing  a  carbon  core  about  24  feet  long,  around 
which  the  material  is  placed  and  heated  by  the  current 


ELECTROCHEMISTRY  179 

passing  through  the  core.     The  furnace  takes  1000  horse- 
power and  the  reaction  is : 

SiO2  +  3C  =  SiC  +  2CO. 
The  material  is  a  mixture  of  sand,  coke,  sawdust  and  salt. 

(c)  Graphite  furnace.     A  resistance  furnace  somewhat 
similar  to  the  carborundum  furnace  is  used,  but  with  lower 
voltage  and  larger  currents;  the  material  is  coke  or  anthra- 
cite, which  by  the  high  temperature  is  converted  into  graph- 
ite,  probably  passing  through  an  intermediate  stage  as  a 
metal  carbide. 

(d)  Silicon  furnace.     Either  arc  or  resistance  furnace 
is  used;  the  reaction  is: 

SiO2  +  2C  =  Si  +  2CO. 
or, 

SiO2  +  2SiC  =  3Si  +  2CO. 

(e)  Titanium  carbide  furnace.     Arc  or  resistance  fur- 
nace is  used  which  requires  a  very  high  temperature ;  that  is, 
a  greater  temperature  than  that  of  the  calcium  carbide 
furnace. 

TiO2  +  3C  =  TiC  +  2CO. 

Other  products  of  the  electric  furnace  are  siloxicon,  sili- 
con monoxide,  etc.,  and  numerous  alloys  of  refractory 
metals,  mainly  with  iron;  as  of  vanadium,  tungsten, 
molybdenum,  titanium,  etc.,  which  are  used  in  steel 
manufacture. 

In  the  steel  industry,  the  electric  furnace  is  finding  a 
rapidly  increasing  use,  in  steel  smelting. 

The  use  of  the  electric  arc  for  the  production  of  nitric 
acid  and  nitrate  fertilizers;  of  the  high-potential  glow  dis- 
charge for  the  production  of  ozone  for  water  purification, 
etc.,  also  are  applications  of  electric  power,  which  are  of 
rapidly  increasing  industrial  importance. 


SIXTEENTH  LECTURE 
THE  INCANDESCENT  LAMP 

The  two  main  types  of  electric  illuminants  are  the  incan- 
descent lamp  and  the  arc  lamp. 

In  the  incandescent  lamp,  the  current  flows  through  a 
solid  conductor  of  practically  constant  resistance,  usually 
in  a  vacuum,  and  the  heat  produced  in  the  resistance  of  the 
conductor  makes  it  incandescent,  thus  giving  the  light. 
Incandescent  lamps  in  an  electric  circuit  thus  act  as  non- 
inductive  ohmic  resistances,  and  as  such  can  be  operated 
equally  as  well  on  constant-potential  as  on  constant-cur- 
rent supply.  As  electric  distribution  systems  are  almost 
always  constant-potential,  most  incandescent  lamps  are 
operated  on  constant-potential,  usually  in  multiple  on 
no-volt  secondary  or  lighting  mains  (one  side  of  an  Edison 
three- wire  circuit).  Only  for  street  lighting,  where  the 
distances  over  which  the  lighting  circuit  extends,  is  too 
great  to  transmit  constant  potential  at  the  low  lamp  voltage 
of  no,  series  connection  of  the  incandescent  lamps  is  used, 
and  the  lamps  then  are  operated  on  constant-current 
direct  or  alternating-current  circuits,  in  the  same  manner 
as  arc  lamps.  However,  even  in  series  lighting  by  incandes- 
cent lamps,  constant-potential  systems  are  increasingly 
applied,  that  is,  a  large  number — 50  to  100  or  more — incan- 
descent lamps  connected  in  series  with  each  other,  directly 
or  through  individual  auto- transformers  or  transformers, 
into  a  constant-potential  supply  circuit,  as  this  offers  a 
higher  efficiency  and  a  much  better  power  factor  of  the 

circuit.     In  this  case,  provisions  have  to  be  made,  either  by 

180 


THE  INCANDESCENT  LAMP  181 

protective  devices  or  by  suitable  design  of  the  auto-trans- 
former, so  that  the  burning  out  or  failure  of  a  lamp  does  not 
affect  the  other  lamps  of  the  series  circuit.  On  constant 
current,  not  infrequently  incandescent  lamps  are  operated 
in  series  with  arc  lamps  on  the  same  circuits,  though  this  is 
not  good  practice,  as  the  incandescent  lamp  is  far  more 
sensitive  to  current  and  voltage  fluctuations  than  the  arc 
lamp,  requiring  a  better  regulation  of  the  circuit,  and  cir- 
cuit fluctuations,  which  would  be  harmless  to  arc  lamps, 
may  seriously  impair  the  life  of  the  incandescent  lamps. 

It  is  now  nearly  40  years  ago  that  the  carbon-filament 
incandescent  lamp  was  industrially  developed  by  Edison, 
with  its  characteristic  features :  carbon  glowing  in  a  vacuum 
perfectly  glass  enclosed  and  with  platinum  leading-in 
wires.  Various  previous  attempts  to  use  platinum  and 
other  materials  as  radiators  had  failed,  due  to  their  lower 
melting  points.  Very  great  differences  were  found  in  the 
efficiency  of  light  production  between  the  filaments  pro- 
duced by  the  carbonization  of  different  vegetable  fibers,  and 
so  an  extensive  search  was  started  for  the  best  fiber,  and  con- 
tinued for  years,  and  expeditions  sent  out  all  over  the  world 
to  find  the  fiber  which  when  carbonized  was  most  stable, 
that  is,  could  be  operated  at  the  highest  temperature  and 
corresponding  efficiency  of  light  production.  This  search 
jended  somewhat  tragically,  for  when  finally  a  bamboo 
fiber  had  been  found  better  than  all  previous  ones,  and 
brought  back  from  the  wilderness,  a  still  better  fiber  had 
just  been  produced  in  the  chemical  laboratory,  of  squirted 
cellulose,  and  from  then  on,  until  in  the  last  years  the 
carbon  filament  became  antiquated,  squirted  cellulose 
fibers  have  exclusively  been  used. 

Pure  cellulose — purified  cotton — is  dissolved,  in  zinc 
chloride  or  in  cupric-ammon,  or  nitrocellulose  in  glacial 


182  GENERAL  LECTURES 

acetic  acid,  and  the  thick  molasses-like  solution  squirted 
through  a  fine  hole,  into  some  liquid,  which  takes  up  the 
solvent  of  the  cellulose,  and  so  hardens  the  fiber:  alcohol 
with  zinc  chloride,  diluted  hydrochloric  acid  with  cupric- 
ammon,  water  with  acetic  acid.  The  horn-like  and  per- 
fectly homogeneous  and  uniform  fiber,  which  is  thus  pro- 
duced, is  then  cut  to  size  and  carbonized.  While  fairly 
efficient,  it  is  not  as  efficient  as  some  forms  of  deposited 
carbon,  and  the  carbonized  cellulose  filaments,  "base 
filaments,"  as  they  are  called,  then  are  heated  by  an  elec- 
tric current  in  gasolene  vapor  under  a  partial  vacuum. 
Thereby  a  thin  layer  of  a  grayish  carbon  is  deposited  on  the 
surface  of  the  filament,  which  is  more  stable,  thus  permits 
operating  such  "treated  filaments"  at  a  higher  efficiency 
than  the  original  base  filaments. 

In  the  few  years  of  Edison's  work  on  the  incandescent 
lamp,  the  carbon-filament  lamp  had  been  developed,  from 
a  conception  ridiculed  by  most  engineers,  to  such  a  perfec- 
tion, that  for  nearly  a  quarter  of  a  century  no  material 
and  radical  further  improvements  were  made,  but  all  the 
advance  made  in  the  carbon-filament  incandescent  lamp, 
from  the  days  when  it  left  Edison's  hands,  until  nearly 
25  years  afterwards,  essentially  consisted  in  a  steady  slow 
improvement  in  manufacturing  details,  resulting  in  a  great 
decrease  of  cost  and  increase  of  uniformity  of  the  product, 
and  as  the  result  thereof  a  slow  and  gradual  increase  of 
efficiency,  up  to  a  specific  consumption  of  3.1  watts  per 
candlepower.  However,  the  improvement  in  the  efficiency 
of  the  incandescent  lamp,  during  the  quarter  of  a  century 
from  the  days  of  jEdison's  work  up  to  the  final  replacement 
of  the  carbon  lamp  by  the  metal-filament  lamp,  was  less 
than  improvements  in  efficiency,  which  since  have  taken, 
place  in  a  single  year  or  two  in  metal-filament  lamps. 


THE  INCANDESCENT  LAMP  183 

Only  one  radical  advance  was  made,  fairly  close  to 
the  last  days  of  the  carbon-filament  lamp,  by  the  develop- 
ment of  a  new  form  of  carbon,  by  the  electrochemical 
research  laboratory  of  the  G.  E.  Co.,  the  so-called  "metallic 
carbon."  This,  produced  from  vapor  deposited  carbon 
at  the  highest  temperature  of  the  electric  furnace,  has 
many  pronounced  metallic  characteristics:  low  resistivity, 
positive  temperature  coefficient,  metallic  luster,  etc., 
and  a  much  higher  stability,  which  permitted  increasing 
the  efficiency  of  light  production,  in  the  so-called  "Gem"1 
lamp,  to  2.5  watts  per  candle.  However,  this  efficiency 
was  not  sufficient  to  save  the  carbon  filament  in  competition 
with  the  much  more  efficient  metal  filaments  of  today,  and 
whether  carbon  as  incandescent  lamp  filament  has  forever 
gone  out  of  use,  or  whether  some  time  some  other  form  of 
carbon  will  be  found,  so  much  more  stable  that  it  can 
compete  with  the  metal-filament  lamps,  remains  in  the 
future. 

The  first  challenge  of  the  supremacy  of  the  carbon- 
filament  incandescent  lamp  came  from  the  Nernst  lamp, 
claiming  an  efficiency  twice  as  high  as  that  of  the  carbon- 
filament  lamp.  Developed  by  Professor  Nernst  in  Germany, 
it  was  introduced  and  industrially  adapted  to  our  country's 
conditions.  The  light-giving  glower  of  the  Nernst  lamp 
operates  in  air,  requiring  and  permitting  no  vacuum.  It 
is  a  short  rod  of  rare  oxides,  similar  to  those  which  constitute 
the  Welsbach  mantle,  which  has  so  greatly  increased  the 
efficiency  of  gas  lighting.  However,  the  Nernst  lamp 
glower  is  not  a  dead  resistance  like  a  carbon  filament  or 
metal  wire,  but  is  a  peculiar  kind  of  conductor,  a  so-called 
' ' pyroelectric  conductor,"  which  has  a  number  of  electrical 
characteristics  similar  to  those  of  the  arc :  it  cannot  be 

1  An  abbreviation  of  "G.  E.  Metallized." 


184  GENERAL  LECTURES 

operated  on  constant  potential  supply,  but  requires  a 
steadying  resistance  or  "ballast  resistance,"  and  it  is  not 
self -starting,  but  has  to  be  started  by  heating,  by  means  of 
a  "heater  spiral."  As  the  result,  the  Nernst  lamp  is  not 
as  simple,  but  must  contain  auxiliary  devices,  ballast 
resistance  and  heater  spiral,  and  mechanism  to  cut  the 
heater  spiral  out  of  circuit  after  the  glower  has  started. 
Herefrom,  and  from  the  high  temperature  of  operation  of 
the  glower,  formidable  difficulties  resulted  in  the  com- 
mercial development  of  the  lamp,  and  when  finally  engi- 
neering skill  had  carried  the  development  so  far  as  to 
commercially  verify  the  claims  of  efficiency,  especially 
in  the  larger  units  of  the  Nernst  lamp,  in  the  meantime  the 
tungsten-filament  lamp  with  its  still  much  higher  efficiency 
had  made  its  appearance,  and  the  Nernst  lamp  met  the 
tragic  fate  of  coming  too  late,  and  now  has  practically 
disappeared. 

The  first  real  metal-filament  lamp  was  the  osmium 
lamp.  It  was  developed  abroad  and  introduced  here  to 
a  limited  extent  only.  It  gave  an  efficiency  of  1.3  to 
1.6  watts  per  candle,  with  good  life.  Industrially,  it 
was  of  no  importance,  as  the  total  amount  of  osmium  on 
earth  would  not  be  sufficient  to  supply  one  year's  demand 
of,  incandescent  lamps.  Theoretically,  however,  it  was 
of  terrible  significance,  by  sounding  the  death-knell  of 
the  carbon  filament,  in  proving  that  metal  filaments  can 
be  operated  at  more  than  twice  the  efficiency  of  the  carbon 
filament,  and  while  osmium  was  not  available  in  sufficient 
quantities,  it  was  not  improbable  that  other  suitable  metals 
might  be  found,  which  are  more  plentiful.  Osmium,  while 
a  metal,  was  never  produced  in  a  ductile  state,  and  the 
osmium  filament  was  made  by  a  squirting  process,  similar 
to  the  carbon  filament. 


THE  INCANDESCENT  LAMP  185 

The  search  for  refractory  metals  then  led  to  the  tantalum- 
filament  lamp,  and  thus  the  tantalum  incandescent  lamp  arose 
and  began  rapidly  to  displace  the  carbon  filament — until  in  its 
turn  it  vanished  before  the  tungsten-filament  lamp  of  today. 

Tantalum  is  not  as  infusible  as  osmium,  and  there- 
fore could  not  offer  quite  as  high  an  efficiency  as  the 
osmium  lamp. '  It  gave,  however,  an  efficiency  of  2  watts 
per  candle,  hence  a  material  advance  even  over  the  metal- 
lized carbon  or  Gem  lamp.  Tantalum  is  a  rare  metal, 
that  is,  it  nowhere  exists  in  large  quantities,  but  it  is 
not  limited  like  osmium,  but  is  found  very  plentifully 
in  small  quantities.  It  is  a  ductile  metal,  can  be  drawn 
into  fine  wire,  and  the  tantalum  lamp  thus  was  the  first 
wire- wound  lamp:  that  is,  the  metal  is  drawn  into  a 
fine  wire  of  the  required  diameter,  the  required  length 
cut  off,  and  then  wound  on  a  framework  in  the  lamp  bulb. 
As  the  result  of  using  drawn  wire,  peculiar  difficulties, 
resulting  in  a  shorter  life  on  alternating-current  circuits, 
occured  by  the  "offsetting"  of  the  tantalum  wire  in  the 
lamp,  after  long  use. 

The  tantalum  lamp  was  invented,  developed  and  manu- 
factured in  Germany;  it  was  introduced  in  this  country, 
and  used  here  for  some  years  in  very  large  quantities.  It 
was  manufactured  in  this  country,  but  from  materials 
imported  from  Germany,  and  complete  manufacture  was 
never  established  here,  as  at  that  time  the  possibilities  of 
the  tungsten  lamp  already  loomed  up  so  strongly,  that 
it  did  not  appear  economical  to  develop  the  manufacture 
of  the  tantalum  lamp  further  than  necessary  to  make  it 
available  as  an  intermediate  step. 

Finally  then  came  the  tungsten  lamp,  or  wolfram  lamp.1 

1  The  chemical  name  of  the  metal  is  wolfram;  "tungsten,"  meaning 
"heavy  stone,"  is  merely  the  name  of  one  of  its  ores,  which  in  this  country 
has  mistakenly  been  applied  to  the  metal. 


186  GENERAL  LECTURES 

While  the  metallized  carbon  filament  (2.5  watts  per  candle) 
was  exclusively  an  American  development,  and  the  Nernst 
lamp  (1.5  to  2.5  watts  per  candle),  the  osmium  lamp  (1.5 
watts  per  candle)  and  the  tantalum  (2  watts  per  candle) 
were  German  developments,  merely  introduced  and  adopted 
in  our  country,  in  the  development  of  the  tungsten  lamp 
America  and  Germany  have  about  equally  shared. 

Originally,  tungsten  did  not  appear  to  be  ductile,  and 
ductile  tungsten  was  produced  only  as  the  result  of  ex- 
tensive research  and  development;  for  years  therefore, 
the  tungsten  filaments  were  made  by  some  squirting 
process,  just  as  the  osmium  and  the  carbon  filaments  had 
been  made.  One  process  consisted  in  squirting  the  fila- 
ment of  tungsten  oxide,  then  reducing  it,  and  purifying  by 
electrically  heating  in  moist  hydrogen.  Another  process 
squirted  the  filament  of  colloidal  metallic  tungsten;  a 
third  process — which  appeared  specially  adaptable  to  larger 
filaments,  due  to  the  low  shrinkage — used  a  mixture  or 
alloy  of  finely  powdered  tungsten  with  several  other 
metals  of  successively  lower  melting  and  boiling  points, 
squirting  such  an  alloy  filament,  and  then  successively 
driving  off  the  alloying  metals  by  electrically  heating. 

Finally  the  research  laboratory  of  the  G.  E.  Co.  de- 
veloped ductile  tungsten,  and  with  this,  all  methods  of 
squirting  naturally  disappeared,  and  the  tungsten  lamp  or 
"mazda  lamp,"  as  it  is  usually  called  by  its  trade  name, 
is  made  from  drawn  tungsten  wire,  which  is  wound  up  on  a 
framework,  similar  as  was  done  with  the  tantalum  lamp. 

As  the  resistance  of  tungsten  is  very  much  lower  than 
that  of  carbon,  and  the  power  consumption,  at  the  same 
candlepower,  much  less,  due  to  its  higher  efficiency,  it 
follows  that  the  tungsten  filament  must  be  very  much 
smaller  and  Ipnger  than  the  carbon  filament  of  the  old 


THE  INCANDESCENT  LAMP  187 

lamp.  Great  difficulties  were  thus  met  with  smaller  units 
of  lamp,  and  with  higher  voltage  as  220,  and  during  the 
days  of  squirted  filaments,  a  number  of  filaments  had  to  be 
used  in  series  in  the  lamp.  This  naturally  increased  the 
cost  of  the  lamp.  The  tungsten  lamp  thus  could  be 
brought  down  to  a  reasonable  price,  made  competitive  with 
the  carbon  lamp  and  the  question  of  free  lamp  renewals 
approached,  only  after  the  solution  of  the  problem  of 
ductile  tungsten. 

The  tungsten  lamp  first  appeared  with  an  efficiency  of 
iJ4  to  i  J^  watts  per  candlepower;  by  gradual  improvement 
in  the  manufacture  of  the  filament  and  in  the  vacuum,  the 
efficiency  was  raised  to  i  watt  per  candlepower,  and 
finally,  by  the  introduction  into  the  lamp  bulb  of  certain 
neutral  gases  of  low  heat  capacity,  at  partial  pressure,  in 
the  so-called  "gas-filled  mazda  lamp"  still  much  higher 
efficiencies  were  made  available,  reaching  in  larger  units 
0.5  and  even  0.45  watts  per  candle. 

With  these  efficiencies,  the  tungsten  lamp  had  become 
superior  not  only  to  all  other  forms  of  incandescent  lamps, 
but  also  more  efficient  than  any  arc  lamp,  with  exception 
of  the  large  units  of  flame  arcs  and  the  luminous  arcs. 
As  neither  of  the  latter  is  suitable  for  indoor  illumination, 
the  use  of  the  arc  for  indoor  illumination  thus  has  entirely 
ceased,  and  the  mazda  lamp  holds  sway  undisputed  in  all 
indoor  lighting. 

The  large  unit  of  the  open  yellow-flame  lamp,  of  the 
short-burning  type,  is  still  superior  in  efficiency  to  the 
large  mazda  lamp,  but  unsuited  for  the  conditions  of  our 
country  due  to  the  required  daily  trimming,  and  the  long- 
burning  yellow-flame  arc  does  not  show  sufficient  superior- 
ity in  efficiency,  which  would  compensate  for  its  lesser 
steadiness,  and  reliability,  and  greater  complication  of 


188  GENERAL  LECTURES 

operation,  while  the  white-flame  arc  lamp  usually  is  infe- 
rior in  efficiency. 

There  is  left  then  as  the  only  remaining  competitor  of  the 
mazda  lamp  in  outdoor  lighting,  the  luminous  arc  or 
magnetite  lamp.  This,  while  about  of  the  same  efficiency 
as  the  large  mazda  lamp,  has  the  disadvantage  of  being 
somewhat  less  simple  in  operation,  requiring  a  constant 
direct-current  circuit,  but  it  has  the  advantage  of  perfectly 
white  light,  while  even  the  gas-filled  mazda  lamp,  though 
the  whitest  of  all  the  incandescent  lamps,  is  still  decidedly 
yellow — so  much  so,  that  to  make  it  white  by  absorbing 
the  excess  of  the  yellow  and  red  rays  in  colored  glasses, 
requires  a  sacrifice  of  nearly  75  per  cent,  of  the  light,  that 
is,  lowers  the  efficiency  to  about  2  watts  per  candle.  For 
many  purposes,  the  yellowish- white  of  the  mazda  lamp  is 
no  disadvantage,  for  some  purposes  it  even  is  an  advantage ; 
it  is,  however,  a  serious  disadvantage  artistically  in  the 
use  of  the  mazda  lamp  for  suburban  street  lighting,  park 
lighting  and  in  general,  wherever  foliage  is  plentiful, 
as  the  .yellowish  light  gives  the  green  foliage  a  faded 
yellowish  look,  and  thereby  spoils  its  appearance,  while 
this  is  not  the  case  with  the  clear  white  light  of  the  luminous 
arc. 

It  appears,  therefore,  that  the  white  luminous  arcs 
(magnetite,  titanium)  are  the  only  illuminants,  which  have 
characteristics  in  efficiency  and  color  value,  that  assure 
their  survival  in  competition  with  the  mazda  lamp,  in 
outdoor  illumination. 

Ductile  tungsten,  as  used  in  the  modern  tungsten  or 
wolfram  or  mazda  lamp,  is  an  extremely  interesting  material. 
It  is  the  heaviest  of  all  known  substances,  is  very  hard  and 
extremely  tough,  can  be  drawn  out  into  very  small  wires, 
so  small  that  several  dozen  of  them  together  are  thinner  than 


THE  INCANDESCENT  LAMP  189 

a  human  hair,  and  fine-drawn  wire  tungsten  is  the  strongest 
of  all  known  substances:  on  thin  tungsten  wires,  tensile 
strength  of  over  400,000  Ib.  per  square  inch  has  been 
observed,  which  is  several  times  as  strong  as  the  strongest 
steel. 

It  is  interesting  to  review  the  various  lamp  filament 
materials:  now,  with  our  present  knowledge,  it  appears 
fairly  obvious,  which  might  be  suitable  as  filaments,  and 
which  not.  Especially  so  with  tungsten:  tungsten  is 
not  a  rare  and  little-known  metal,  but  is  extensively  used 
in  the  industries  since  a  long  time.  As  alloying  material 
in  steel,  it  gives  hardness  and  toughness  and  special  mag- 
netic qualities,  which  led  to  its  use  in  magnet  steels, 
while  the  salts  of  tungsten  have  been  and  are  used  very 
extensively  for  fireproofing  inflammable  fabrics,  as  theater 
decorations.  As  filament  material  it  had  been  considered 
in  the  early  days,  but  failed  by  too  low  a  melting  point, 
due  to  lack  of  purity:  it  required  the  highest  chemical  art 
to  produce  tungsten  so  pure  as  to  give  the  required  ex- 
tremely high  melting  point,  as  already  an  extremely 
small  quantity  of  foreign  material  as  carbon,  destroys  its 
usefulness.  For  instance,  tungsten  carbide  is  relatively 
very  fusible,  and  an  addition  of  i  per  cent,  of  tungsten 
carbide,  to  the  metallic  tungsten,  would  seriously  lower  its 
melting  point.  However,  tungsten  carbide,  CW2,  consists 
of  96.9  per  cent,  tungsten  and  3.1  per  cent,  carbon,  and 
the  presence  of  0.03  per  cent,  of  carbon  in  the  tungsten 
metal,  would  already  give  a  contamination  of  i  per  cent, 
of  carbide,  and  thereby  affect  its  properties  unfavorably. 

If  all  the  chemical  elements  are  arranged  in  order  of 
their  atomic  weights  in  a  table,  they  give  the  so-called 
"periodic  system  of  elements,"  that  is,  all  the  character- 
istic properties  of  the  elements  are  represented  by  their 


190  GENERAL  LECTURES 

arrangement  in  this  table.  We  find  then  two  points 
or  " poles"  of  volatility,  opposite  each  other,  one,  at  helium 
and  hydrogen,  non-metallic  elements,  and  the  other  one, 
at  mercury,  metallic  elements.  Opposite  to  each  other  and 
to  the  poles  of  volatility,  we  find  two  poles  of  refactori- 
ness:  the  non-metallic  pole,  at  carbon — surrounded  by 
boron,  silicon,  etc. — and  the  metallic  pole,  at  tantalum, 
tungsten,  osmium,  iridium. 

The  choice  of  filament  material  thus  limits  itself  to  these 
two  points  in  the  system  of  elements:  carbon  and  its 
immediately  adjacent  elements,  and  tungsten  and  its 
immediately  adjacent  metals.1 

Of  all  the  elements,  carbon  is  the  most  refractory,  with  a 
melting  point  probably  above  4ooo°C.  Tungsten  is  next, 
melting  at  3  4oo°C.  Thus  on  first  sight  we  should  expect  that 
carbon,  with  its  higher  melting  point  than  tungsten,  should 
give  a  higher  efficiency  of  incandescence,  by  permitting 
operation  at  higher  temperature.  So  it  would  be,  if  carbon 
could  be  operated  as  near  to  its  melting  point  as  tungsten. 
This,  however,  is  not  the  case,  and  the  melting  point  alone 
does  not  determine  the  permissible  operating  temperature 
of  the  lamp  filament,  but  the  evaporation  of  the  material 
below  the  melting  point  is  equally  determining. 

All  materials  evaporate  already  below  their  boiling 
points,  and  even  below  their  melting  point.  Thus  ice  and 
snow  gradually  evaporate.  Now  with  some  materials, 
as  tungsten,  evaporation  below  the  melting  point  is  very 
small,  so  that  the  temperature  can  be  raised  very  close 

1  It  is  interesting  to  note  that  the  melting  point  rises  from  the  relatively 
low  one  of  platinum,  to  iridium  and  still  higher  osmium,  and  on  the  other 
side  rises  from  tantalum  to  tungsten,  as  the  highest.  A  still  higher  melt- 
ing point  thust  might  be  expected  from  the  unknown  element  which  fills 
the  gap  between  tungsten  and  osmium,  the  second  higher  homologue  of 
manganese,  if  it  exists,  and  this  should  be  superior  even  to  tungsten,  as 
lamp  filament. 


THE  INCANDESCENT  LAMP  191 

to  the  melting  point,  within  a  few  hundred  degrees,  before 
appreciable  evaporation  occurs.  Other  materials  again 
evaporate  very  materially  even  considerably  below  .the 
melting  point,  such  as  camphor  for  instance.  Carbon 
belongs  in  the  latter  class,  and  though  its  melting  point  is 
above  40oo°C.  at  2ooo°C.  carbon  evaporates  already  more 
than  tungsten — with  its  lower  melting  point,  at  34oo°C. — 
evaporates  at  3ooo°C.  As  the  carbon  vapor  deposits  on 
the  glass  bulb,  and,  carbon  being  black,  blackens  the  bulb, 
and  the  loss  of  material  reduces  the  size  of  the  carbon 
filament,  the  high  rate  of  evaporation  of  carbon  at  tempera- 
tures far  below  the  melting  and  boiling  points,  thus  limits 
the  permissible  operating  temperature  and  thereby  the 
available  efficiency  of  the  carbon  filament  to  much  lower 
values  than  given  by  metals,  which,  though  they  melt 
at  lower  temperature  than  carbon,  have  a  much  lower 
evaporation  or  vapor  tension. 

The  reason  may  probably  be  found  in  the  size  of  the 
atom:  carbon  has  the  atomic  weight  12,  tantalum,  tungsten 
and  osmium  the  atomic  weights  185,  187  and  191,  and  the 
light  carbon  atom  might  be  expected  to  separate  from  the 
structure  far  easier  than  the  metal  atom  of  a  weight  more 
than  15  times  as  heavy. 

This  also  explains  the  great  differences  in  filament 
efficiencies,  found  with  different  modifications  of  carbon: 
the  very  dense  and  solid  metallic  carbon  of  the  gem  lamp 
naturally  may  be  expected  to  evaporate  less,  at  the  same 
temperature,  than  the  porous  and  amorphous  carbon  of  the 
base  filament,  and  with  the  same  evaporation  and  thereby 
same  lamp  life,  the  former  thus  could  be  operated  at 
materially  higher  temperature  and  thereby  efficiency  of 
light  production. 

The  foremost  advantage  of  the  metals  as  filament  ma- 


192  GENERAL  LECTURES 

terials  in  incandescent  lamps,  which  made  the  great  increase 
of  efficiency  of  light  -production  possible,  thus  is  not  the 
higher  melting  point  of  the  metals,  but  is  the  low  vapor 
tension  of  these  heavy  metals  close  up  to  the  melting 
point,  which  made  it  possible  to  take  advantage  in  the 
operating  temperature  of  the  high  melting  points,  while 
with  carbon  the  high  vapor  tension  at  relatively  low  tem- 
perature made  it  impossible  to  take  advantage  of  the  high 
melting  point. 

As  the  evaporation  increases  with  decreasing  gas  pressure 
in  the  lamp  bulb,  apparently  the  use  of  a  vacuum  should  be  a 
disadvantage,  and  the  filament  evaporates  less,  that  is, 
lasts  longer  or  permits  higher  operating  temperature  and 
thus  higher  radiation  efficiency,  if  operated  under  consider- 
able gas  pressure.  With  the  filament  operating  in  a  per- 
fect vacuum,  all  the  electric  power  put  into  the  lamp  is 
radiated  from  the  filament  as  visible  or  invisible  light 
(ultrared  radiation),  and  the  higher  the  temperature,  the 
greater  is  the  radiation  efficiency,  that  is,  percentage  of  visible 
rays,  and  therefore  also  the  efficiency  of  the  lamp.  If, 
however,  now  the  lamp  bulb  is  filled  with  a  gas,  with  the 
same  electric  power  supplied  to  it,  the  temperature  of  the 
filament  is  very  greatly  lowered,  as  a  large  part  of  the 
energy  is  conducted  away  by  the  gas,  and  carried  away  by 
gas  currents.  To  maintain  the  same  filament  temperature 
and  thereby  the  same  light  production,  more  energy  there- 
fore is  required  in  the  gas-filled  lamp,  than  in  the  vacuum 
lamp,  and  the  former  thus  has  less  efficiency,  at  the  same 
filament  temperature.  However,  the  reduction  of  filament 
evaporation  by  the  gas  pressure  permits  increasing  the  fila- 
ment temperature — by  still  further  increasing  the  power 
supply — and  thereby  increasing  the  radiation^efficiency,  that 
is,  the  percentage  of  radiation,  which  is  visible  as  light,  and 


THE  INCANDESCENT  LAMP  193 

thus  useful,  and  the  question  then  is,  which  of  the  two 
effects  predominates :  the  loss  of  efficiency  by  the  waste  of 
energy  by  convection  and  conduction  by  the  gas,  or  the 
gain  in  efficiency,  by  the  possibility  of  operating  the  fila- 
ment at  a  higher  temperature. 

The  loss  of  energy  in  the  gas  depends  on  the  size  of  the 
gas-washed  filament  surface,  compared  with  its  volume, 
and  in  small  lamp  units,  due  to  the  relatively  large 
filament  surface,  gas  filling  offers  no  advantage,  but  rather 
materially  lowers  the  efficiency.  In  larger  units,  however, 
of  100,  300  and  500  watts,  the  use  of  a  thick  filament, 
coiled  in  a  spiral  in  a  small  space,  has  made  it  possible  to 
reduce  the  loss  of  heat  from  the  filament  surface,  by  con- 
vection and  conduction  through  the  gas,  so  much,  that 
the  increased  operating  temperature  of  the  filament  gives 
a  substantial  increase  of  efficiency,  and  in  this  manner 
it  has  been  possible  in  the  last  years,  to  more  than  double 
the  efficiency  of  light  production  in  the  larger  lamp  units, 
in  the  so-called  " gas-filled  mazda  lamp.'1  Most  advanta- 
geous obviously  is  a  gas  of  the  lowest  heat  conduction 
and  heat  capacity,  as  nitrogen  or  argon,  while  hydrogen 
for  instance,  with  its  high  heat  conductivity  and  high  spe- 
cific heat,  is  harmful  under  all  conditions.  The  amount  of 
gas  is  such  as  to  give  approximately  atmospheric  pressure 
at  the  operating  temperature  of  the  lamp,  as  safest. 

In  discussing  efficiencies,  it  must  be  realized  that  the 
incandescent  lamp  inherently  has  no  definite  efficiency,  but 
its  efficiency  varies  with  the  power  supply,  or  with  the 
impressed  voltage,  and  can  be  made  almost  anything. 

Thus  for  instance  a  mazda  lamp,  at  25  watts  power 
input,  no  volts,  may  give  25  candles,  that  is,  an  efficiency 
of  i  watt  per  candle. 

But  if  we  put  35  watts  into  this  lamp,  by  raising  the 

13 


194  GENERAL  LECTURES 

voltage  to  130,  we  get  70  candles,  or  twice  the  previous 
efficiency,  J^  watt  per  candle,  and  if  we  lower  the  power 
supply  to  20  watts,  by  lowering  the  voltage  to  100,  we 
get  only  10  candles,  corresponding  to  an  efficiency  of  2 
watts  per  candle. 

However,  at  i  watt  per  candle,  the  lamp  will  last  an 
average  of  1000  hours,  while  at  the  higher  efficiency  of 
J^  watt  per  candle  it  would  last  only  about  100  hours, 
and  at  the  lower  efficiency  of  2  watts  per  candle  will  proba- 
bly last  5000  hours. 

Thus,  the  "efficiency"  of  an  incandescent  lamp  has  a 
meaning  only  when  related  to  its  life,  and  any  efficiency 
can  be  secured  by  sacrificing  the  life.  Inventors  have  not 
always  realized  this,  and  so  have  deceived  themselves  in 
believing  that  they  had  discovered  a  wonderful  improve- 
ment, by  forgetting  the  essential  influence  of  the  life  on 
the  efficiency. 

The  purpose  of  the  lamp  is  to  give  light,  and  the  lamp  thus 
will  operate  at  its  highest  efficiency,  when  producing  the 
light  at  the  lowest  cost  per  candle-hour. 

The  cost  of  light  production  by  the  incandescent  lamp, 
per  candle-hour,  consists  of  the  cost  of  power  and  the  cost 
of  lamp  renewal.  Increasing  the  voltage,  decreases  the 
cost  of  power,  but  shortens  the  life  and  thereby  increases 
the  cost  of  renewal,  per  lamp-hour,  and  the  reverse  is  the 
case  with  a  decrease  of  voltage.  There  must  thus  be  a 
compromise  between  cost  of  power  and  cost  of  renewal, 
at  which  the  total  cost  is  a  minimum,  that  is,  the  economic 
efficiency  of  the  lamp  a  maximum. 

Supposing  at  no-volt  supply,  a  mazda  lamp  consumes 
25  watts,  gives  25  candles,  or  i  watt  per  candle,  and  has 
a  life  of  1000  hours.  Its  total  light  output  then  is  25,000 
candle-hours.  It  consumes  during  its  life  25  kilo  watt -hours 


THE  INCANDESCENT  LAMP  195 

and  at  a  cost  of  8  cents  per  kilowatt-hour,  and  50  cents  per 
lamp  renewal,  the  total  cost  of  the  light  given  by  the  lamp 
would  be:  25  X  8  +  50  =  $2.50,  that  is,  10  cents  per  1000 
candle-hours. 

Suppose  now  we  operate  this  lamp  at  130  volts.  Then 
it  consumes  35  watts,  gives  70  candles,  or  J^  watt  per 
candle,  but  lasts  only  100  hours.  Its  total  light  output 
thus  is  only  7000  candle-hours.  The  total  power  consump- 
tion is  3.5  kilowatt-hours  hence  the  total  cost  of  operation 
3.5  X  8  -f-  50  =  78  cents,  or  11.4  cents  per  1000  candle- 
hours.  That  is,  though  we  have  doubled  the  efficiency  of 
light  production  of  the  lamp,  the  cost  of  the  light  per  lamp- 
hour  has  increased,  by  the  decreased  length  of  lamp  life. 

Suppose  now  we  operate  this  lamp  at  100  volts.  Then 
it  consumes  20  watts,  gives  10  candle,  or  2  watts  per  candle, 
but  lasts  5000  hours.  The  total  light  output  then  is 
50,000  candle-hours.  The  total  cost  of  operation  is  100 
X  8  +  50  =  $8.50,  or  17  cents  per  1000  candle-hours,  that 
is,  a  materially  higher  price  for  the  light,  showing  that  it 
would  be  very  uneconomical  to  operate  the  lamp  at  such 
low  rating,  as  to  give  a  very  long  life. 

With  different  costs  of  power,  and  different  renewal 
costs,  obviously  somewhat  different  results  are  derived 
regarding  to  the  most  efficient  industrial  life  and  the 
proper  efficiency  of  operation.  However,  as  industrial 
conditions  do  not  vary  over  such  a  great  range,  and  such 
efficiency-life  curves  are  fairly  flat,  it  is  possible  to  strike 
an  average  which  fairly  well  satisfies  average  industrial  con- 
ditions of  the  use  of  the  incandescent  lamp.  If  then  we 
speak  of  the  efficiency  of  an  incandescent  lamp,  we  always 
mean,  or  should  mean,  the  industrial  efficiency,  that  is,  the 
efficiency  which  giving  minimum  cost  per  candle-hour,  con- 
sidering cost  of  power  as  well  as  cost  of  renewal. 


196  GENERAL  LECTURES 

With  the  carbon -filament  lamp,  this  problem  of  lamp 
efficiency  had  been  studied  for  many  years,  and  found 
that  a  useful  life  of  500  hours  is  the  most  efficient,  that  is, 
under  the  "efficiency"  of  a  carbon-filament  incandescent 
lamp  always  was  understood  the  efficiency  which  give  to 
the  lamp  an  average  life  of  500  hours:  at  500  hours,  the 
lamp  either  was  burned  out,  or  by  blackening  of  the  bulb 
the  efficiency  so  much  lowered  as  to  make  the  lamp  of  no 
further  use.  This  latter  maximum  permissible  decrease 
of  efficiency  during  useful  life  had  been  fixed,  by  similar 
consideration,  as  20  per  cent,  that  is,  burn  out  or  decrease 
of  light  production  to  80  per  cent,  of  the  initial  terminated 
the  useful  life  of  the  carbon  lamp. 

Incidentally,  it  is  of  interest  to  notice,  that  blackening 
of  the  bulb  as  cause  of  gradual  deterioration  has  prac- 
tically disappeared  in  the  metal-filament  lamp,  and  already 
had  largely  decreased  in  the  metallized  carbon  or  gem 
lamp :  these  lamps  usually  end  their  life  either  by  burn  out 
of  the  filament,  or  by  impairment  of  the  vacuum,  and  in 
the  latter  case,  blackening  of  the  bulb  occurs  in  a  few 
hours,  terminating  the  life,  but  the  gradual  blackening, 
through  hundreds  of  hours,  is  absent. 

As  the  power  consumption  of  the  mazda  lamp  is  less, 
the  cost  of  renewal  somewhat  higher,  the  same  consideration 
of  the  proper  useful  life,  in  relation  to  the  efficiency,  which 
give  500  hours  with  the  carbon  lamp,  have  led  to  the  re- 
quirement of  a  useful  life  of  1000  hours  with  the  mazda 
lamp,  and  1500  hours  with  the  series  street-lighting  mazda 
lamp,  and  these  values  seem  to  very  fairly  satisfy  the  in- 
dustrial conditions. 

Obviously,  where  the  cost  of  power,  etc.,  is  very  different, 
materially  different  conclusions  regarding  the  proper  useful 
life  result.  This  for  instance  is  the  case  with  the  small 


THE  INCANDESCENT  LAMP  197 

mazda  lamps  used  in  pocket  flash  lights.  In  these,  the 
power  is  derived  from  dry  cells,  at  a  cost  which  probably 
is  between  $10  and  $50  per  kilowatt-hour.  At  this  enor- 
mous power  cost,  even  with  the  small  lamp  unit  and  there- 
fore small  power  consumption,  the  industrial  efficiency 
maximum  lies  at  a  useful  life  of  a  few  hours  only,  that  is, 
it  is  more  economical  to  save  power  by  running  the  lamp 
at  very  high  efficiency. 

Similar  considerations  apply  also  to  automobile  lamps, 
kinematoscope  lamps,  etc.,  in  which  a  materially  shorter 
industrial  life  than  1000  hours  is  economical. 

As  regards  to  the  meaning  of  candlepower,  for  many 
years,  during  the  days  of  the  carbon  filament,  a  somewhat 
fictitious  rating  was  used,  by  a  "horizontal  candlepower," 
which  was  only  about  80  per  cent,  of  the  true,  average  or 
mean  spherical  candlepower,  that  is,  the  candlepower  which 
characterizes  the  total  flux  of  light  given  by  the  lamp. 
The  ratio  of  the  nominal  or  horizontal  candlepower,  to 
the  true  or  mean  spherical,  was  then  measured  and  called 
the  "spherical  reduction  factor,"  and  usually  varied  be- 
tween 0.77  and  0.82. 

However,  with  the  general  introduction  of  the  metal- 
filament  lamp — in  which  very  often  the  spherical  reduction 
factor  is  practically  unity,  that  is,  the  horizontal  candle- 
power  equal  to  the  mean  spherical — the  use  of  the  term 
horizontal  candlepower  is  rapidly  disappearing.  Further- 
more, a  lamp  rating  in  watts  is  becoming  more  customary. 


SEVENTEENTH  LECTURE 
ARC  LIGHTING 

While  incandescent  lamps  can  be  operated  on  constant 
potential  as  well  as  on  constant  current,  the  arc  is  essentially 
a  constant-current  phenomenon.  At  constant  length, 
the  voltage  consumed  by  the  arc  decreases  with  increase 
of  current,  as  shown  by  curve  I  in  Fig.  47.  If,  therefore, 
an  attempt  is  made  to  operate  such  an  arc  on  constant 
potential,  for  instance  on  80  volts — which  would  correspond 
to  3.9  amperes  on  curve  I — then  any  tendency  of  the  current 
to  increase — as  by  a  momentary  drop  of  the  arc  resistance- 
would  lower  the  required  arc  voltage,  and  so  increase  the  cur- 
rent, at  constant  supply  voltage,  hence  still  further  lower 
the  arc  voltage,  etc.,  and  a  short-circuit  would  result.  Vice 
versa,  a  momentary  decrease  of  arc  current,  by  requiring 
more  voltage  than  is  available,  would  still  further  decrease 
the  current,  increase  the  required  voltage,  etc.,  and  the 
arc  would  extinguish. 

Therefore  only  such  apparatus  is  operative  on  constant 
potential,  in  which  an  increase  of  current  requires  an  in- 
crease of  voltage,  and  vice  versa;  and  so  limits  itself. 

While,  therefore,  arcs  can  be  operated  on  a  constant- 
current  system,  to  run  arc  lamps  on  constant  potential, 
some  current-limiting  device  is  necessary  in  series  with  the 
arc,  as  a  resistance;  or,  in  an  alternating-current  circuit, 
a  reactance. 

The  supply  voltage  required  to  operate  the  arc  consuming 
the  voltage  represented  by  curve  I  must  therefore  be  higher 

than  that  given  by  this  voltage,  and  must  be  at  least  as 

198 


ARC  LIGHTING 


199 


high  as  that  given  by  the  curve  II.  The  latter  thus  is 
called  the  stability  curve  of  the  arc.  Thus,  for  instance, 
at  4  amperes,  the  arc  cannot  be  operated  at  less  than  104 
volts  supply.  At  104  volts  supply  the  limit  of  stability 


FIG.  47. 

is  reached;  and  for  supply  voltages  higher  than  104,  the 
arc  is  stable,  the  more  so,  the  higher  the  supply  voltage 
is  above  104.  The  difference  in  voltage  between  the  supply 
voltage  and  the  arc  voltage  is  consumed  by  the  "steadying 
resistance"  of  the  arc. 


200  GENERAL  LECTURES 

High  reactance  in  series  with  the  direct-current  arc 
retards  the  current  fluctuations  and  so  reduces  them;  so 
that  with  reactance  in  series  to  the  direct-current  arc, 
the  arc  can  be  operated  by  a  supply  voltage  closer  to  the 
stability  curve  II  than  without  reactance;  reactance 
therefore  is  very  essential  in  the  steadying  resistance 
of  a  direct-current  arc.  Obviously,  no  series  reactance 
can  enable  operation  of  the  arc  I  on  a  supply  voltage 
below  that  given  by  the  stability  curve  II. 

Constant-potential  arc  lamps  are,  therefore,  necessarily 
less  efficient  than  constant-current  arc  lamps,  due  to  the 
power  consumed  in  the  steadying  resistance.  A  large 
part  of  this  power  is  saved  in  alternating  constant-potential 
arc  lamps,  by  using  reactance  instead  of  resistance,  but  the 
power  factor  is  therefore  greatly  lowered,  and  the  constant- 
potential  alternating  arc  lamp  rarely  has  a  power  factor 
of  over  70  per  cent. 

Where  therefore  high-potential  constant-current  circuits 
are  permissible,  as  for  outdoor  or  street  lighting,  arc  lamps 
are  usually  operated  on  a  constant-current  circuit,  with 
series  connection  of  from  50  to  100  lamps  on  one  circuit. 
With  the  exception  of  a  few  of  the  larger  cities,  all  the 
street  lighting  by  arc  lamps  in  this  country  is  done  by  con- 
stant-current systems,  either  direct  current  or  alternating 
current. 

For  direct  constant-current  supply,  arc-light  machines 
have  been  built,  and  a  few,  of  the  Brush  type,  are  still 
used.  In  these  machines,  inherent  regulation  for  constant 
current  is  produced  by  using  a  very  high  armature  reaction 
and  relatively  weak  field  excitation;  that  is,  the  armature 
ampere-turns  are  nearly  equal  and  opposite  to  the  field 
ampere-turns,  and  thus  both  very  large  compared  with  the 
difference,  the  resultant  ampere-turns,  which  produce 


ARC  LIGHTING  201 

the  magnetic  field.  A  moderate  increase  of  current  and 
consequent  increase  of  armature  ampere-turns  therefore 
greatly  reduces  the  resultant  ampere-turns  and  so  the  field 
magnetism  and  the  voltage,  that  is,  the  machine  tends  to 
regulate  for  constant  current.  Perfect  constant-current 
regulation  then  is  secured  by  some  governing  device,  as 
an  automatic  regulator  varying  a  resistance  shunted 
across  the  series  field.  It  must,  however,  be  understood 
that  the  "regulator"  of  the  arc  machine  does  not  give  a 
constant-current  regulation,  but  the  armature  reaction 
of  the  machine  does  this,  and  the  regulator  merely  makes 
it  perfect;  but  even  with  the  regulator  disconnected,  arc 
machines  give  fairly  close  constant-current  regulation. 

With  the  development  of  the  mercury-arc  rectifier, 
which  converts  constant  alternating  current  into  constant 
direct  current,  arc  machines  have  gone  out  of  use,  as  the 
mercury-arc  rectifier  in  combination  with  the  stationary 
constant-current  transformer  enables  us  to  derive  constant 
direct  current  from  the  alternating-current  constant-poten- 
tial supply  system,  without  moving  machinery. 

Constant  alternating  current  is  derived  by  a  constant-cur- 
rent transformer  or  constant-current  reactance.  Diagram- 
matically,  the  constant-current  transformer  is  shown  in 
Fig.  48.  The  primary  coil  P  and  the  secondary  coil  5  are 
movable  with  regard  to  each  other  (which  of  the  two  coils 
is  movable,  is  immaterial,  or  rather,  is  determined  by  con- 
sideration of  design).  Fig.  48  shows  the  coil  5  suspended 
and  its  weight  partially  balanced  by  counterweight  W. 

With  the  secondary  coil  5  close  to  the  coil  P,  that  is,  in 
the  lowest  position,  most  of  the  magnetism  produced  by  the 
primary  coil  P  passes  through  the  secondary  coil  S,  and 
the  secondary  voltage  therefore  is  a  maximum.  The  fur- 
ther the  secondary  coil  moves  away  from  the  primary  coil, 


202 


GENERAL  LECTURES 


the  more  of  the  magnetism  passes  between  the  coils,  the  less 
through  the  secondary  coil,  and  the  lower  therefore  is  the 
secondary  voltage,  which  becomes  a  minimum  (or  zero,  if 
so  desired),  with  the  secondary  coil  at  a  maximum  dis- 
tance from  the  primary,  that  is,  in  the  top  position. 

Primary  current  and  secondary  current  are  proportional 
and  in  opposition  to  each  other,  and  repel  each  other,  and 
the  repulsion  is  proportional  to  the  product  of  the  two 
currents ;  that  is,  proportional  to  the  square  of  the  secondary 


A 

T 

< 

r  i 

*- 

I  l" 

\ 

w 

1  L 

•*- 
f  r 

~t" 

•-H  -^ 

-fftf 

>  ^>   i- 

—  *• 

^3          V-  V>                v^ 

1  1  1  1 

FIG.  48. 

current.  The  weight  of  the  secondary  coil  is  balanced  by 
the  counterweight  W  and  the  repulsion  from  the  primary 
coil,  at  normal  secondary  current.  Any  increase  of  sec- 
ondary current  by  a  decrease  of  load,  increases  the  repulsion, 
in  this  way  pushing  the  secondary  coil  further  away  from 
the  primary  and  thereby  reducing  the  secondary  voltage 
and  thus  the  current ;  and  vice  versa,  a  decrease  of  secondary 
current,  by  an  increase  of  load,  reduces  the  repulsion  and 
so  causes  the  secondary  coil  to  come  nearer  to  the  primary, 
that  is,  increases  its  voltage  and  so  restores  the  current. 


ARC  LIGHTING  203 

Such  an  arrangement  regulates  for  constant  current  be- 
tween the  voltage  limits  given  by  the  two  extreme  positions 
of  the  movable  coil.  These  usually  are  chosen  from  some 
margin  above  full  load,  down  to  about  one-third  load. 

The  constant-current  reactance  operates  on  the  same 
principle:  the  two  coils  P  and  5  are  connected  in  series 
with  each  other  into  the  arc  circuit  supplied  from  the 
constant-potential  source,  and  by  separating  or  coming  to- 
gether, vary  in  reactance  with  the  load,  and  thereby  main- 
tain constant  current. 

While  the  alternating-current  arc  lamp  is  less  efficient,  that 
is,  gives  less  light  for  the  same  power,  than  the  direct-cur- 
rent arc  lamp,  the  disadvantages  of  the  use  of  numerous  arc 
machines  had  led  to  the  extended  adoption  of  alternating- 
current  series  arc  lighting  before  the  development  of  the 
mercury-arc  rectifier,  which  enabled  the  operation  of  direct- 
current  arc  circuits  from  constant-current  transformers. 
Thousands  of  such  enclosed  series  alternating  arc  lamps 
are  still  in  use  all  over  the  country,  though  they  have  long 
ceased  to  have  any  right  of  existing,  as  they  are  very  much 
inferior  in  efficiency  to  the  luminous  arc  or  magnetite,  and 
to  the  mazda  incandescent  lamp :  the  latter  giving  three  or 
more  times  as  much  light  for  the  same  power  consumption. 

The  carbon  arc  lamp  of  old  has  for  over  a  generation 
dominated  all  outdoor  or  street  lighting,  and  much  of  the 
indoor  lighting:  indeed  practically  all  lighting  of  larger 
areas,  as  halls,  assembly  places,  etc.  It  originated  even 
before  the  carbon-filament  incandescent  lamp,  represented 
a  much  larger  unit  of  light,  and  gave  a  much  higher  ef- 
ficiency, so  that  during  the  days  of  the  carbon-filament 
incandescent  lamp,  wherever  a  large  unit  of  light  could  be 
used,  the  arc  lamp  found  its  proper  field.  By  the  metal- 
filament  incandescent  lamp,  and  especially  the  mazda  lamp, 


204  GENERAL  LECTURES 

the  plain  carbon  arc  lamp  has,  however,  become  outclassed 
so  enormously,  that  it  is  only  a  historical  curiosity  today. 

In  the  plain  carbon  arc  lamp  practically  all  light  comes 
from  the  incandescent  tips  of  the  carbons,  very  little  from 
the  arc  flame.  Once  more,  then,  by  using  materials  in  the 
arc-lamp  electrodes,  which  in  the  arc  flame  give  an  intensely 
luminous  spectrum,  the  arc  lamp  was  made  competitive 
with  the  newer  metal-filament  incandescent  lamps,  by  add- 
ing the  large  amount  of  light  given  by  the  luminous  arc 
flame,  and  so  greatly  increasing  the  efficiency  of  the  arc 
lamp  in  the  flame  arc  lamp  and  the  luminous  arc  lamp. 

So  far  only  three  materials  have  been  found,  which  in 
luminous  arcs  give  efficiencies  vastly  superior  to  incandes- 
cence: mercury,  calcium  (lime),  and  titanium. 

The  mercury  arc  has  the  advantage  of  perfect  steadiness, 
a  long  life — requiring  no  attention  for  thousands  of  hours— 
and  high  efficiency  over  a  fairly  wide  range  of  candle- 
powers  ;  but  it  is  seriously  handicapped  for  many  purposes 
by  its  bluish-green  color. 

In  the  flame  carbon  lamp  carbons  impregnated  with 
calcium  compounds,  usually  calcium  fluoride,  borate,  etc., 
are  used,  and  the  arc  then  has  an  orange-yellow  color. 
The  compounds,  after  coloring  the  arc  and  giving  it  effi- 
ciency, escape  as  smoke;  the  arc  therefore  must  be  an  open 
arc,  or  provisions  made  to  condense  and  collect  the 
deposit. 

The  open  arc  lamp,  which  was  used  in  the  early  days, 
had,  however,  been  almost  entirely  superseded  by  the  en- 
closed carbon  arc,  in  spite  of  the  somewhat  lower  efficiency 
of  the  latter ;  and  the  inconvenience  of  daily  attendance 
required  by  an  open  arc,  and  the  large  consumption  of 
carbons,  makes  a  return  to  this  type  impracticable.  For 
this  reason  the  open-flame  carbon  lamp  has  not  proven 


ARC  LIGHTING  205 

suitable  for  general  outdoor  illumination,  as  street 
lighting. 

Enclosed-flame  carbon  arc  lamps  thus  were  designed, 
in  which  by  a  circulating  system  between  a  double  set  of 
glass  globes  the  smoke  is  deposited  at  some  place  where 
it  does  not  obstruct  the  light,  and  such  "long  burning," 
or  "enclosed"  or  "regenerative"  flame  carbon  lamps,  with 
yellow  color  of  light,  have  been  fairly  successful  and  found 
an  extensive  introduction  in  street  lighting.  But  in  effi- 
ciency they  were  somewhat  inferior  to  the  open  or  short- 
burning  flame  arc,  and  thus,  when  the  gas-filled  mazda 
lamp  entered  the  field  with  J£  watt  efficiency,  or  better, 
whatever  little  superiority  in  efficiency  may  still  have  ex- 
isted in  the  large-flame  arc,  did  not  appear  sufficient  to 
compensate  for  the  lesser  steadiness  and  greater  complica- 
tion of  operation,  and  thus  the  enclosed  yellow-flame  lamp 
is  rapidly  going  out  of  use,  in  competition  with  the  gas-filled 
mazda  lamp.  The  white-flame  carbon  arc  always  has  been 
inferior  in  efficiency  to  the  yellow-flame,  and  thus  has 
never  found  an  extensive  use  on  the  basis  of  efficiency  com- 
petition, but  has  merely  been  used  to  a  limited  extent, 
due  to  its  color  value,  and  in  this  respect  it  has  never  made 
much  headway  against  the  luminous  arc  or  magnetite 
lamp,  which  in  steadiness,  efficiency  and  constancy  of  the 
color  is  superior  to  the  white-flame  carbon  lamp. 

The  only  type  of  arc  lamp,  which  thus  has  been  able  to 
hold  and  extend  its  field  against  the  competition  of  the 
mazda  incandescent  lamp,  is  the  luminous  arc,  in  which 
carbon  has  entirely  been  eliminated  from  the  electrodes, 
and  the  light-giving  electrode  consists  of  the  oxides  of  iron, 
titanium  and  chromium,  with  small  quantities  of  alkali 
fluorides  as  steadier  of  the  arc  flame:  the  intermediate 
oxide  of  iron,  magnetite,  is  the  arc  conductor,  and  composes 


206  GENERAL  LECTURES 

the  major  part  of  the  electrode.  The  oxide  of  titanium — 
rutile — is  the  light-giver,  therefore  used  in  as  large  a  per- 
centage as  practicable  without  interfering  with  steadiness 
and  conductivity,  and  the  oxide  of  chromium,  used  in  a 
lesser  percentage,  is  the  steadier  or  restrainer:  it  increases 
the  steadiness  of  the  flame,  and  increases  the  life  of  the 
electrode,  thus  giving  electrode  life  of  150  to  300  hours. 
However,  in  too  large  percentage,  chromium  again  lowers 
the  efficiency,  and  its  use  is  thereby  limited. 

In  commercial  efficiency,  the  luminous  arc  is  about 
equal  to  the  highest  efficiencies  of  the  gas-filled  mazda  lamp ; 
in  operation  it  is  cheaper  as  regards  to  the  renewal  of 
electrodes,  but  as  arc  lamp,  on  a  constant  direct-current 
system  through  mercury-arc  rectifiers,  it  is  somewhat  more 
complicated,  requiring  more  attention  than  the  operation 
of  gas-filled  mazdas  on  a  constant-potential  series  system. 
In  color,  however,  the  luminous  arc  is  perfectly  white, 
and  therefore  has  the  advantage  wherever  foliage,  trees 
and  plants  are  within  the  rays  of  the  light,  and  their 
appearance  is  of  interest,  as  the  luminous  arc  does  not  give 
the  faded  and  dead  look  to  foliage,  which  the  yellow  carbon 
arc  as  well  as  the  yellow  incandescent  lamp,  even  still  the 
gas-filled  mazda  lamp,  gives.  It,  therefore,  appears,  that 
both  types  of  outdoor  illuminants,  the  mazda  incandescent 
lamp  and  the  magnetite  arc  lamp,  will  retain  legitimate 
fields  of  application,  while  for  all  indoor  illumination,  the 
mazda  lamp  entirely  covers  the  field.  The  development  of 
an  alternating-current  luminous  arc  lamp,  that  is,  a  lamp 
which  could  be  operated  directly  from  a  constant  alternat- 
ing-current circuit  without  rectifier,  would  still  further 
extend  the  field  of  the  luminous  arc  in  outdoor  illumination, 
if  of  competitive  efficiency,  and  such  a  lamp  probably  will 
sometime  make  its  appearance. 


ARC  LIGHTING  207 

In  the  arc  lamp,  the  current  is  carried  across  the  gap 
between  the  terminals  by  a  stream  of  vapor  of  the  electrodes ; 
thus  the  electrodes  consume  more  or  less  rapidly.  Some 
feeding  mechanism  is  therefore  required  to  move  the  elec- 
trodes toward  each  other  during  their  consumption.  This 
arc  lamp  mechanism  may  be  operated  by  the  current,  or 
by  the  voltage,  or  by  both.  This  gives  the  three  different 
types  of  lamps:  the  series  lamp,  the  shunt  lamp,  and  the 
differential  lamp. 

In  the  series  lamp,  an  electromagnet  energized  by  the 
lamp  current,  and  balanced  against  a  weight  or  a  spring, 
moves  the  carbons  toward  each  other  when  by  their 
burning  off,  the  arc  lengthens  and  the  current  decreases. 
Obviously,  this  lamp  cannot  be  used  on  constant-current 
circuits,  or  with  several  lamps  in  series,  but  only  as  single 
lamp  on  constant-potential  circuits. 

In  the  shunt  lamp,  the  controlling  magnet  is  shunted 
across  the  arc,  and  with  increasing  arc  length  and  con- 
sequent arc  voltage,  moves  the  electrodes  toward  each 
other.  In  constant-current  circuits,  this  lamp  tends  to- 
ward hunting,  and  therefore  requires  a  very  high  reactance 
in  series ;  it  thereby  gives  a  lower  power  factor  in  alterjiating- 
current  circuits,  and  has  therefore  been  superseded  by  the 
differential  lamp.  It  has,  however,  the  advantage  of  not 
being  sensitive  to  changes  of  current. 

In  the  differential  lamp,  an  electromagnet  in  series  with 
the  arc  opposes  an  electromagnet  in  shunt  to  the  arc,  and 
the  lamp  regulates  for  constant  arc  resistance.  It  was  the 
lamp  most  universally  used  in  constant-potential  and 
constant-current  systems,  as  most  stable  in  its  operation; 
but  in  constant-current  systems,  it  requires  that  the  current 
be  constant  within  close  limits:  if  the  current  is  low,  the 
arc  is  too  short,  and  the  lamp  gives  very  little  light,  and 


208  GENERAL  LECTURES 

if  the  current  is  high,  the  arc  becomes  so  long  as  to  endanger 
the  lamp. 

From  the  operating  mechanism  the  motion  is  usually 
transmitted  to  the  electrode  by  a  clutch,  which  releases  and 
lets  the  electrodes  slip  together. 

In  the  carbon  arc  lamp  of  old,  the  mechanism  was 
" floating;"  that  is,  the  upper  carbon,  held  by  the  opposing 
forces  of  shunt  and  series  magnets,  moves  with  every 
variation  of  the  arc  resistance,  and  so  maintains  very  closely 
constant  voltage  on  the  arc.  In  the  long-burning  luminous 
arc,  as  the  magnetite  lamp,  the  light  comes  from  the  arc 
flame,  and  thus  constant  length  of  arc  flame  is  required  for 
constant  light  production.  The  floating  mechanism,  which 
constantly  varies  the  arc  length  with  the  variation  of  the 
arc  resistance,  has  therefore  been  superseded  by  a  mech- 
anism which  sets  the  arc  at  fixed  length,  and  leaves  it  there 
until  with  the  consumption  of  the  electrodes  the  arc  has 
sufficiently  lengthened  to  cause  the  shunt  coil  to  operate 
and  to  reset  the  arc  length.  Thus  in  some  respects, 
these  lamps  are  shunt  lamps. 

During  the  early  days  of  the  open  carbon  arc  lamp,  9.6, 
6.6  and  4  amperes  were  the  currents  used  in  direct-current 
arc  circuits,  with  about  40  volts  per  lamp.  The  4 -ampere 
arc  very  soon  disappeared,  as  giving  practically  no  light. 

In  the  enclosed  arc  lamp,  the  carbons  are  surrounded  by 
a  nearly  air-tight  globe,  which ,  restricts  the  admission  of 
air  and  thus  the  combustion  of  the  carbon,  and  so  increases 
the  life  of  the  carbons  from  8  or  10  hours  to  70  to  120  hours. 
In  these  lamps,  lower  currents  and  higher  arc  voltages,  that 
is,  longer  arcs,  are  used:  in  direct-current  circuits,  6.6 
amperes  and  5  amperes,  with  70  to  75  volts  per  lamp; 
in  alternating-current  circuits,  7.5  and  6.6  amperes  are  used 
with  the  same  arc  voltage. 


ARC  LIGHTING  209 

In  the  direct-current  magnetite  arc  lamp,  4  amperes,  6.6 
amperes  and  sometimes  5  amperes,  and  75  to  80  volts  per 
lamp  are  used. 

In  the  application  to  outdoor  lighting,  both  types  of 
lamps,  the  luminous  arc  and  the  mazda  incandescent, 
can  obviously  be  applied  in  the  same  manner :  either,  in  the 
usual  form  of  street  lighting,  as  fairly  large  units,  250  to  500 
watts,  on  high  poles  and  with  considerable  spacing  along 
the  sides  of  the  street,  or  where  trees  abound,  preferably 
in  the  middle  of  the  street,  or,  in  so-called  ornamental 
lighting,  on  ornamental  standards,  with  underground  cable 
supply,  along  the  two  sides  of  the  street  in  business 
thoroughfares,  as  so-called  "white  way  lighting,"  or  in 
smaller  units  in  so-called  "ornamental"  or  "boulevard 
lighting,"  in  the  middle  of  the  boulevard  in  grass  and  flower 
plots.  In  the  latter  case,  the  mazda  lamp  has  the  advantage 
that  more  frequent  smaller  units  can  be  used,  while  the 
ornamental  magnetite  lamp,  with  300  watt  power  consump- 
tion, requires  a  wider  spacing,  but  on  the  other  hand  by  its 
white  color  of  light  gives  a  better  appearance  of  the  lawns, 
trees  and  foliage  in  general. 


EIGHTEENTH  LECTURE 

MODERN  POWER  GENERATION 
AND  DISTRIBUTION 

With  the  development  of  electrical  engineering,  elec- 
tricity is'  more  and  more  becoming  the  universal  form 
of  power,  supplying  the  energy  demand  of  modern  civili- 
zation, and  the  various  local  power  stations,  electric 
generating  stations,  etc.,  are  rapidly  being  replaced  by 
substations,  receiving  the  power  from  huge  electric  gener- 
ating stations  or  groups  of  such  stations,  over  a  system  of 
high-power  feeders,  and  converting  it  into  whatever  form 
of  energy  is  demanded :  electrical,  mechanical,  chemical,  etc. 

Thus  power  generation  has  become  a  separate  industry, 
distinct  from  the  use  of  power. 

In  these  huge  electric  power  stations,  of  hundred  thou- 
sands of  kilowatt  capacity,  experience  has  shown  three 
features  as  essential  for  successful  operation. 

i.  All  electrical  apparatus  must  be  operated  in  parallel 
on  the  same  set  of  busbars,  not  only  the  generators  in  one 
station,  but  where  a  number  of  stations  feed  into  the  same 
system,  they  must  operate  in  parallel  over  tie  cables, 
thereby  essentially  giving  one  set  of  busbars,  interconnect- 
ing all  the  generating  stations,  as  a  ring  bus — as  shown  dia- 
grammatically  by  the  heavy  black  lines  in  Fig.  50.  Where 
two  frequencies  are  used,  on  the  generators,  as  60  and  25 
cycles,  they  are  preferably  synchronized  with  each  other 
through  frequency  changers,  to  secure  the  interchange  of 

power  between  them. 

210 


MODERN  POWER  GENERATION  211 

2.  The  generating  system  must  be  capable  of  unlimited 
expansion,  by  extension  of  the  power  houses  or  the  joining 
into  the  system  of  additional  power  houses,  without  any 
increase   of   the   risk   and   danger   of   operation,    that   is, 
without  reducing  the  reliability  of  operation  or  increasing 
the  chances  for  trouble.     Therefore : 

3.  The  control  of  the  system  must  be  such   that  the 
maximum  amount  of  power,  which  can  be  let  loose   de- 
structively in  case  of  accident,  at  any  point  of  the  system, 
is  not  increased  by  an  increase  of  the  size  of  the  system, 
and  is  limited  to  a  value,  which  can  under  emergency  con- 
dition safely  be  handled  and  controlled  by  modern  circuit- 
opening  and  controlling  devices. 

I  in  Fig.  49  shows  diagrammatically  an  arrangement  of 
such  high-power  stations,  containing  six  generators,  6*1 
to  GV 

The  generators  usually  are  high-voltage  three-phase 
turbo-alternators,  of  10,000  to  50,000  kilowatt  capacity, 
feeding  directly  into  the  power  distribution  feeders  at 
about  10,000  volts  (6000  to  15,000  volts).  In  the  larger 
units  of  steam-turbine  alternators,  such  very  high  efficien- 
cies have  been  reached  on  the  electrical  as  well  as  on  the 
steam  end,  that  there  appears  very  little  probability,  at 
least  for  a  long  time  to  come,  that  any  form  of  internal 
combustion  engine  should  approach  the  efficiencies  of 
steam-turbine  electric  power  generation,  and  the  steam 
turbine  thus  has  become  the  universal  source  of  primary 
power,  and  will  undoubtedly  remain  it  for  the  next  future. 

Assuming  the  generators  G  in  Fig.  49  to  be  20,000  kilowatt 
units.  In  case  of  accidental  short-circuit  at  the  generator 
terminals,  the  only  limitation  of  the  momentary  short- 
circuit  current  is  the  internal  self-inductive  reactance  of 
the  generator,  usually  of  about  3  to  4  per  cent,  in  turbo- 


212 


GENERAL  LECTURES 


alternators.  That  is,  at  the  rated  current,  the  internal 
reactance  consumes  3  to  4  per  cent,  of  the  rated  voltage, 
and  at  short-circuit  current,  where  the  total  machine 
voltage  is — in  the  first  instance,  that  is,  before  armature 


FIG.  49. 

reaction  comes  into  play — consumed  by  the  self -inductive 
reactance,  the  short-circuit  current  would  thus  be  33  to  25 
times  the  rated  current,  and  with  six  generators  directly 
connected  to  the  busbars,  a  short-circuit  at  the  busbars 


MODERN  POWER  GENERATION  213 

would  let  loose  a  momentary  current  200  to  150  times  that 
of  one  generator,  giving  a  momentary  maximum  power  of 
2,000,000  to  1,500,000  kilowatts.1  Obviously,  no  circuit- 
breakers  or  other  devices  of  a  size  such  as  to  permit  their 
economical  use  in  every  generator,  can  control  such  enormous 
power. 

The  power  which  the  generators  can  feed  into  the 
system  is  therefore  limited  by  "power-limiting  reactances," 
inserted  into  the  generator  leads,  as  shown  in  Fig.  49  as 
GRi  to  GR6. 

Fig.  49  as  diagrammatical  representation  shows  one  phase 
only:  obviously  three  reactances,  etc.,  are  required  at 
every  generator.  These  generator  reactances  are  treated 
as  a  part  of  the  generator,  and  the  generator  circuit  breakers 
thus  located  between  the  reactances  GR  and  the  busbars  B. 

In  general,  in  large  high-power  generating  systems, 
the  power-limiting  generator  reactances  are  chosen  so  as  to 
limit  the  momentary  (symmetrical)  generator  short-circuit 
current  to  about  10  times  the  generator  current,  that  is, 
a  total  generator  reactance  of  8  to  12  per  cent,  is  aimed  at. 
Assuming  an  internal  reactance  in  the  generators  of  4  per 
cent.,  the  external  reactances  GR  thus  would  be  designed 
for  4  to  8  per  cent,  reactance. 

The  size  of  such  power-limiting  reactances  is  conveniently 
expressed  in  per  cent.;  an  8  per  cent,  generator  power- 
limiting  reactance  thus  means  a  reactance  having  a  terminal 
voltage  equal  to  8  per  cent,  of  the  generator  voltage,  at  the 
rated  generator  current. 

But  even  with  the  maximum  short-circuit  current  of  the 
generators  limited  by  generator  reactance  GR,  with  increas- 
ing size  of  the  system,  and  thus  increasing  number  of  genera- 

1  The  maximum  power  of  an  alternator  may  approximately  be  con- 
sidered as  open-circuit  volts  times  short-circuit  current  divided  by  2. 


214  GENERAL  LECTURES 

tors  feeding  into  the  busbars,  soon  conditions  are  reached 
giving  a  destructive  accumulation  of  power  at  the  busbars 
in  case  of  a  short-circuit.  Assuming  for  instance  in  Fig. 
49 :  three  power-house  feeding  into  the  busbars,  i  with  6 ; 
ii  with  3;  and  in  again  with  six  generators  of  20,600 
kilowatts  each,  or  a  total  of  300,000  kilowatts — which  is 
less  than  some  existing  stations.  With  the  short-circuit 
current  limited  to  10  times  the  rated  current,  by  the 
generator  reactance,  the  maximum  short-circuit  current 
would  correspond  to  3,000,000  kilowatts,  and  the  maximum 
power  available  at  a  short  at  the  busbars,  might  reach 
1,500,000  kilowatts,  which  is  beyond  the  capacity  of 
circuit-controlling  devices. 

Thus  a  further  limitation  of  power  became  necessary  in 
these  •  huge  systems,  by  the  use  of  busbars  reactances, 
shown  in  Fig.  49  as  BRi,  BR2,  BRZ.  That  is,  the  busbar  is 
divided  into  sections,  which  are  joined  by  reactances  BR. 
No  interference  with  parallel  operation  occurs  by  these 
busbar  reactances — unless  they  are  of  excessive  reactance — 
but  they  limit  the  power  which  can  be  let  loose  at  any 
busbar  section,  and  thereby  permit  indefinite  increase  of  the 
system,  without  increasing  danger  from  possible  accumu- 
lation of  power. 

As  each  busbar  section,  BI,  B2,  #3,  etc.,  receives  power 
from  generators,  and  sends  power  out  into  feeders,  the 
flow  of  power  along  the  busbars,  over  the  busbar  reactances 
BR,  is  only  the  difference  between  the  power  generated  and 
that  consumed  in  any  busbar  section,  and  by  reasonable 
care  in  choosing  the  number  of  generators  operating  at 
each  busbar  section,  the  exchange  of  power  over  the  busbar 
reactances  can  be  kept  very  moderate:  theoretically,  to  less 
than  one-quarter  the  output  of  one  generator. 

It   is   therefore   permissible,    and   desirable,    to   choose 


MODERN  POWER  GENERATION  215 

higher  values  in  the  busbar  reactances,  than  in  the  generator 
reactances,  and  from  20  to  30  per  cent,  reactances  are  not 
infrequently  used  in  the  busbars. 

A  20  per  cent,  reactance  would  mean,  that  the  potential 
difference  across  this  reactance,  when  traversed  by  the  rated 
current  of  one  generator,  is  20  per  cent,  of  the  rated  voltage. 
In  case  of  a  dead  short-circuit  on  one  busbar  section,  and 
with  full  voltage  maintained  at  the  adjoining  busbar  section, 
the  busbar  reactances  would  have  to  absorb  the  total  volt- 
age, thus,  as  20  per  cent,  reactance,  carry  5  times  the 
rated  current  of  a  generator,  or  both  busbar  reactances 
together  (at  either  end  of  the  short-circuited  busbar  section) 
10  times  the  generator  current.  If  then  each  busbar 
section  contains  three  generators,  limited  to  10  times  their 
rated  current  at  short-circuit,  the  maximum  short-circuit 
current  of  such  a  busbar  section  would  be  that  coming 
from  three  generators  and  two  busbar  reactances,  or  40 
times  the  rated  current  of  a  generator,  giving  a  maximum 
possible  power  development  at  the  busbar,  of  400,000 
kilowatts.  This  is  within  the  range  of  emergency  capacity 
of  modern  high-power  circuit-breakers,  thus  can  be  con- 
trolled safely — though  obviously  not  without  the  circuit- 
breakers  showing  distress  of  overload,  throwing  oil,  and 
requiring  cleaning  up  after  the  operation. 

It  must  be  realized  that  even  with  considerable  current 
flowing  over  a  busbar  reactance,  and  thus  considerable 
potential  difference  at  the  reactor  terminals,  this  does  not 
mean  a  drop  of  voltage  and  thus  a  voltage  difference  be- 
tween adjoining  busbar  sections.  As  the  reactive  voltage 
is  in  quadrature  with  the  main  voltage,  it  is  absorbed  by  a 
phase  difference  between  the  (equal)  voltages  of  adjoining 
busbars.  For  instance,  even  with  full  rated  generator 
current  flowing  over  a  20  per  cent,  busbar  reactance,  the 


216  GENERAL  LECTURES 

reactive  voltage  of  20  per  cent,  would  be  taken  care  of  by 
the  terminal  voltages  of  the  adjoining  busbar  section  being 
out  of  phase  by  20  per  cent,  that  is,  displaced  in  phase  by 
2  sin  0/2  =  0.20,  or  by  12°,  in  other  words  by  such  a  small 
angle  as  not  to  affect  the  operation  regarding  stability,  etc. 
This  phase  displacement  obviously  would  be  brought 
about  by  increasing  the  generator  excitation  in  the  busbar 
section  which  sends  out,  reducing  it  in  the  section  which 
receives  power  from  the  adjoining  busbar  section,  adjusting 
both  excitation  so  as  to  have  the  same  voltage  in  both 
busbar  sections. 

The  use  of  busbar  reactances  made  it  permissible  to  tie 
together  the  various  power  houses  of  the  system,  into  one 
unified  system,  by  tie  cables  TC  and  H TC,  as  shown  in 
Fig.  49.  Conveniently,  such  tie  cables  between  two 
power  houses  would  be  the  location  of  a  busbar  dividing 
reactance,  BR2  and  BRS ,  and  if  possible,  this  reactance 
would  be  divided  in  two,  and  half  located  at  either  end  of 
the  cable,  so  as  to  separate  a  breakdown  of  the  cable  from 
the  busbars. 

Obviously,  several,  two  to  six  or  more,  cables  would  as 
a  rule  be  used  in  multiple  in  the  tie  lines  between  the 
stations,  and  where  the  distance  between  the  stations  is 
large,  as  between  i  and  in,  step-up  transformers  T  in- 
serted between  the  tie  cable  and  the  station,  that  is,  with 
io,ooo-volt  generation  and  distribution,  a  20,000  or  30,000- 
volt  tie  line  or  ring  cable  used  between  the  stations,  to 
join  them  together. 

From  the  busbar  sections  then  branch  off  the  power 
feeders  F,  through  their  circuit-breakers  FC,  as  diagrammat- 
ically  shown  in  Fig,  49. 

Reactances  in  the  feeders  ' 'feeder  reactances,"  as 
shown  as  FC  in  Fig.  49,  are  not  always  used,  and  are  not  so 


MODERN  POWER  GENERATION  217 

essential,  since  the  limitation  of  the  power  in  the  busbars, 
by  generator  and  busbar  reactances,  also  limits  the  power 
in  the  feeder  short-circuit.  However,  breakdowns  in 
feeders  are  very  much  more  frequent  than  in  the  station, 
and  as  a  feeder  short-circuit  close  to  the  generating  station 
is  practically  at  the  busbar — if  no  feeder  reactance  is  used— 
and  the  power  concentrated  in  a  busbar  short-circuit  is 
still  very  large,  it  is  desirable,  and  good  practice  tends 
more  and  more  to  the  use  of  feeder  reactances  also.  Usu- 
ally somewhat  smaller  reactances  are  used,  as  4  per  cent. 
However,  a  4  per  cent,  feeder  reactance  means  much  more 
reactance  than  an  8  per  cent,  generator  reactance,  as  the 
former  refers  to  the  current  rating  of  the  feeder,  which  is 
very  much  less  than  that  of  the  generator.  Thus  a  4  per 
cent,  feeder  reactance  would  in  general  reduce  the  effect  of 
a  short-circuit  in  the  feeder  near  the  generating  station, 
to  a  small  fraction  of  the  jolt  which  would  result  without 
the  feeder  reactance. 

It  is  obvious  that  various  arrangements  and  modifi- 
cations can  be  made  in  the  size  and  proportioning,  location, 
etc.,  of  reactances  and  other  controlling  elements,  fulfilling 
the  same  general  purpose,  and  diagram  Fig.  49  thus  is 
essentially  only  illustrative.  Thus  for  instance,  a  different 
arrangement  of  feeders  is  shown  in  station  n  of  Fig.  49: 
groups  of  feeders  joined  together  on  small  feeder  buses  FB, 
and  the  latter  connected  to  the  main  bus  by  feeder  reactance 
FR  and  emergency  feeder  circuit-breaker  FBC. 

Furthermore,  local  and  industrial  limitations,  as  space 
and  location,  etc.,  may  modify  the  layout  of  the  station, 
and  require  serious  study  to  secure  the  required  safety 
within  the  local  limitations. 

As  seen,  the  safety  of  the  system  depends  entirely  on  the 
reliability  of  the  power-limiting  reactances:  generators 


218  GENERAL  LECTURES 

may  burn  out  and  circuit-breakers  fail,  without  giving 
more  than  local  trouble,  as  long  as  the  power-limiting 
reactances  block  the  extension  of  the  trouble. 

Thus  the  utmost  reliability  of  the  power-limiting  re- 
actances is  of  the  foremost  importance,  and  they  should 
be  the  last  structural  element,  where  cheapness  is  con- 
sidered. 

Furthermore,  the  service  of  the  reactances  is  specially 
severe,  since  it  is  not  under  their  normal  operating  con- 
ditions, but  under  abnormal  conditions,  under  enormous 
overstress,  that  they  are  called  upon  to  fulfill  their  functions 
of  safeguard:  under  short-circuit,  when  traversed  by  ten 
times  their  normal  current,  that  is,  exposed  to  100  times 
the  normal  mechanical  stresses  between  the  conductors,  ex- 
posed to  100  times  the  normal  izr  heating,  then  their  re- 
liability is  essential  for  the  system.  Also,  as  inductances 
interconnected  between  cables,  static  discharges  and  im- 
pulses are  liable  to  pick  out  the  reactances,  and  if  once  an 
impulse  flashes  over  between  turns  and  short-circuits,  the 
reactance  ceases  to  be  a  reactance,  fails  in  its  function. 
Thus  in  the  design  of  such  reactance,  safety  of  the  systems 
which  are  to  be  protected,  requires  that  every  consideration 
should  be  given  to  their  utmost  reliability:  they  must  con- 
tain nothing  which  might  possibly  give  way,  break  or  get 
loose  even  under  enormous  mechanical  overstresses ;  there 
must  be  nothing  inflammable  or  combustible,  which  might 
be  damaged  by  heat;  there  must  be  no  metal  or  other 
conductor,  whether  insulated  or  grounded,  near  the  cir- 
cuit of  the  reactance,  to  which  electrostatic  sparks  could 
flash  and  start  a  short-circuit  between  turns;  in  short 
they  should  be  the  most  reliable  of  all  the  station  parts. 

Such  power-limiting  reactance  as  a  rule  must  be  air 
reactance,  that  is,  cannot  contain  an  iron  core,  as  the  use 


MODERN  POWER  GENERATION  219 

of  the  latter  would  make  the  reactance  uneconomically 
large,  due  to  the  very  low  magnetic  density  required  in  the 
iron.  In  a  10  per  cent,  reactance,  under  short-circuit, 
that  is,  under  the  condition  under  which  the  reactance  is 
needed  and  therefore  must  be  fully  there,  the  magnetic 
density  is  ten  times  the  normal,  as  the  current  is  increased 
tenfold,  and  as  in  the  former  case  the  iron  must  be  below 
magnetic  saturation — otherwise  the  reactor  would  lose  its 
reactance  just  when  it  is  needed — it  follows  that  normally 
a  density  would  have  to  be  used,  so  low  as  to  be  hopelessly 
uneconomical  in  iron.  With  a  4  per  cent,  reactance,  this 
would  be  still  worse,  as  it  would  have  to  operate  at  less 
than  4  per  cent,  of  magnetic  saturation.  This  is  the  reason 
why  in  all  such  power-limiting  reactances  air  cores  or  the 
equivalent  (concrete  cores)  are  used,  and  iron  excluded. 

In  general,  such  high-power  generating  systems  are 
either  25  cycles  or  60  cycles;  in  the  larger  systems  25 
cycles  are  usually  preferred,  but  the  extensive  use  of  60 
cycles  led  to  the  introduction  of  6o-cycle  generators  in 
addition  to  the  2  5 -cycle  ones,  so  that  the  modern  very 
high-power  generating  systems  contain  25  cycles  and  some 
6o-cycle  alternators,  with  frequency  changers  interlocking 
the  two  systems  and  providing  for  interchange  of  power. 

The  distribution  of  power  from  these  huge  power- 
generating  stations  of  Fig.  49,  then  is  shown  diagram- 
matically  in  Fig.  50. 

Gi,  G2  and  Gs  show  three  generating  stations,  G\  and  Gz 
containing  six  generators  each,  in  two  sections  divided  by  a 
busbar  reactance,  and  G%  contains  one  section  of  three 
generators,  thus  a  total  of  fifteen  generators  in  five  sections 
of  60,000  kilowatts  rated  capacity,  each.  The  three  sta- 
tions are  joined  in  a  common  ring  bus,  shown  in  heavy 
drawn  line,  consisting  of  underground  cable  between  the 


220 


GENERAL  LECTURES 


stations:  low-tension  cable  between  the  adjoining  stations 
GI  and  G%,  and  high-tension  cable  fed  through  transformers, 
between  the  distant  station  £3,  and  the  other  two  stations. 


FIG.  50. 

From  these  generating  stations  are  fed  a  number  of 
substations,  denoted  by  numbers  i  to  20,  through  power 
feeders,  each  substation  usually  receiving  two  to  four 
feeders,  as  shown  in  Fig.  50.  The  different  feeders  of  the 
same  substation  are  preferably  derived  from  different  bus- 


MODERN  POWER  GENERATION  221 

bar  sections  or  even  power  houses,  to  increase  the  relia- 
bility of  operation,  that  is,  maintain  power  on  the  sub- 
station, even  if  a  busbar  section  or  even  an  entire  power 
house  should  go  out  of  service.  As  far  as  possible,  how- 
ever, for  convenience  of  control  and  reliability  of  operation, 
the  different  substations  are  fed  from  separate  feeders,  so 
that  trouble  in  one  substation  is  less  liable  to  involve  other 
substations,  and  troubles  in  a  feeder  extends  to  its  sub- 
station only.  However,  this  is  not  always  economically 
feasible,  and  sometimes  two  or  more  substations  are  con- 
nected to  the  same  feeders,  as  shown  in  Fig.  50.  To  some 
extent,  substations  may  be  tied  together  by  tie  cables, 
shown  dotted  in  Fig.  50.  This  is  a  great  economic  advan- 
tage, in  saving  cable  capacity  by  permitting  interchange 
of  power  between  substations,  but  it  makes  the  control  of 
the  system  far  more  difficult,  especially  when  carried  out  to 
a  considerable  extent,  and  no  perfectly  satisfactory  device 
has  yet  been  developed  to  overcome  this  disadvantage. 

In  the  substations,  the  power  supplied  from  the  generat- 
ing stations  then  is  transformed  down  to  22oo-volt  three 
or  four-wire,  for  general  alternating-current  distribution  for 
lighting  and  power,  or,  if  the  generating  stations  are  25 
cycles,  the  power  is  converted  by  frequency  changers  from 
25-cycle  supply  to  6o-cycle  distribution.  Direct-current 
three-wire  converters  supply  power  for  three-wire  under- 
ground Edison  direct-current  distribution;  6oo-volt  syn- 
chronous converters  feed  railway  systems ;  industrial  power, 
factories  and  mills  are  supplied  by  other  substations;  in 
short,  these  substations  i  to  18  fulfill  all  the  functions  of  the 
electrical  generating  stations  of  former  times,  and  indeed 
very  often  have  been  generating  stations,  which,  with  the 
erection  of  the  unified  high-power  central  stations,  were 
changed  to  substations. 


222  GENERAL  LECTURES 

In  the  densely  populated  ''metropolitan"  district,  M  of 
Fig.  50,  distribution  is  essentially  by  underground  cables, 
at  10,000  volts,  more  or  less,  through  large  substations 
from  which  all  the  power  demands  of  the  territory  are 
supplied. 

In  the  less  densely  populated  territory  throughout  the 
country,  and  the  State,  the  distribution  more  commonly 
used  is  overhead,  frequently  at  3o,ooo-volt  isolated  delta, 
as  a  very  convenient  voltage,  and  when  well-built  and 
well-insulated,  most  reliable  in  operation.  Such  systems, 
as  shown  at  A,  B  and  C,  then  tie  into  the  metropolitan 
system  M  by  feeders,  with  transformers  located  at  the  place 
of  change  from  underground  to  overhead.  The  substations, 
a,  b  *  *  *  k,  then  are  similar  distribution  centers  as  i, 
2  *  *  *  1 8  in  the  metropolitan  territory,  different  only  in 
size  and  character  of  load  as  depending  on  the  local  require- 
ments. Commonly,  they  have  been  generating  systems, 
feeding  smaller  cities,  villages,  etc.,  and  often  they  still 
contain  some  of  the  generating  machinery  as  reserve  and 
for  emergency  use,  or  for  use  during  peak  loads. 

In  such  outlying  feeders  and  circuits,  usually  a  steam- 
turbine  sustaining  station  is  provided,  as  shown  at  gi,  £2, 
etc.,  that  is,  at  one  of  the  larger  stations  near  the  end  of  the 
circuit,  a  turbo-alternator  of  a  few  thousand  kilowatts  is 
installed  and  controls  and  sustains  voltage  and  power. 

Where  water  power  exists,  water-power  stations  may  be 
included  in  such  network  of  outlying  distribution,  as  at 
HI,  and  further-distant  water  powers  feed  into  the  system, 
as  at  H2.  Also,  long-distance  water-power  lines  may  tie 
into  the  main  generating  station,  as  shown  at  HZ,  and  these 
water  powers  may  then  be  used  either  for  supplying 
whatever  power  they  may  happen  to  have  available :  much 
in  spring  and  fall,  possibly  nothing  in  summer;  or  they 


MODERN  POWER  GENERATION  223 

may  be  merely  reserve  or  standby  connections,  permitting 
the  system  to  draw  power  from  the  water  power,  when 
much  power  is  needed,  or  when  an  accident  has  disabled  a 
part  of  the  steam  plant,  or  to  supply  the  customers  of  the 
hydraulic  plant  from  the  steam-turbine  station  in  case  of 
the  failure  or  deficiency  of  the  hydraulic  plant. 

It  is  obvious  that  a  variety  of  arrangements  and  methods 
of  installation  and  operation  is  feasible,  depending  on  the 
local  conditions,  the  relative  amounts  of  concentrated 
metropolitan  load  and  of  distributed  rural  load,  etc.,  in 
organizing  the  modern  system  of  general  power  generation 
and  distribution  to  substations  which  take  the  place  of  the 
smaller  generating  stations  of  former  times,  but  as  part  of 
the  huge  system,  share  in  the  higher  economy  of  power 
production,  of  administration  and  of  operation,  of  the 
larger  unified  system. 


APPENDIX  I 

EFFECT  OF  ELECTRICAL  ENGINEERING 
ON  MODERN  CIVILIZATION 

(From  an  address  before  the  Franklin  Institute,  1914) 
I 

The  use  of  electricity  in  modern  civilized  life  is  rapidly  in- 
creasing: in  lighting  our  homes,  factories,  streets;  in 
industrial  power  applications ;  in  domestic  service,  from  the 
fan  motor  to  the  electric  bell  or  the  heating  and  cooking 
device;  in  transportation,  while  no  great  inroads  have  yet 
been  made  into  the  field  of  the  steam  locomotive,  an 
entire  system  of  electric  railroads  has  sprung  up  all  over  the 
country,  fully  comparable  in  size  and  power  demand  with 
the  steam  railway  system;  large  new  industries  have 
developed  in  electrochemistry  and  electrometallurgy,  sup- 
plying us  with  materials  unavailable  before — as  alumi- 
num— or  improving  the  production  of  other  materials— 
as  copper  refining,  etc. 

All  these  applications  are  uses  of  energy.  In  nearly  all, 
electrical  energy  is  replacing  some  other  form  of  energy  used 
heretofore:  chemical  energy  of  fuel,  or  mechanical  energy 
of  steam  or  gas  engines,  etc. 

To  understand  the  reasons  which  enable  electrical  energy 
to  compete  successfully  with  other  forms  of  energy,  which 
are  longer  and  more  familiarly  known,  we  have  to  look  into 
its  characteristics. 

Electrical  energy  can  be  transported — or,  as  we  usually 

call  it,    transmitted — economically   over  practically    any 
15  225 


226  GENERAL  LECTURES 

distance.  Mechanical  energy  can  be  transmitted  over  a 
limited  distance  only,  by  belt  or  rope  drive,  by  compressed 
air,  etc. ;  heat  energy  may  be  carried  from  a  central  steam 
heating  plant  for  some  hundred  feet  with  moderate  efficiency, 
but  there  are  only  two  forms  of  energy  which  can  be  trans- 
mitted over  practically  any  distance — that  is,  which  in  the 
distance  of  transmission  are  limited  only  by  the  economical 
consideration  of  a  source  of  energy  nearer  at  hand :  electrical 
energy,  and  the  chemical  energy  of  fuel.  These  two  forms 
of  energy  thus  are  the  only  competitors  whenever  energy 
is  required  at  a  place  distant  from  any  of  Nature's  stores 
of  energy.  Thus,  when  in  the  study  of  a  problem  of 
electric  power  transmission  we  consider  whether  it  is  more 
economical  to  transmit  power  electrically  from  the  water 
power  or  the  coal  mine,  or  generate  the  power  by  a  steam 
plant  at  the  place  of  demand,  both  really  are  transmission 
problems,  and  the  question  is  whether  it  is  more  economical 
to  carry  energy  electrically  over  the  transmission  line, 
or  to  carry  it  chemically,  as  coal  by  the  railroad  train  or 
boat,  from  the  source  of  energy  supply  to  the  place  of 
energy  demand,  where  the  energy  is  converted  into  the 
form  required,  as  into  mechanical  energy  by  the  electric 
motor  or  by  steam  boiler  and  engine  or  turbine. 

Electrical  energy  and  chemical  energy  both  share  the 
simplicity  and  economy  of  transmission  or  transportation, 
but  electrical  energy  is  vastly  superior  in  the  ease,  simplicity, 
and  efficiency  of  conversion  into  any  other  form  of  energy, 
while  the  conversion  of  the  chemical  energy  of  fuel  into 
other  forms  of  energy  is  difficult,  requiring  complicated 
plants  and  skilled  attendants,  and  is  so  limited  in  efficiency 
as  to  make  the  chemical  energy  of  fuel  unavailable  for  all 
but  very  restricted  uses:  heating,  and  the  big,  high-power 
steam  plant.  Pressing  the  button  turns  on  the  electric 


ELECTRICAL  ENGINEERING  227 

light  and  thereby  starts  conversion  into  radiating  energy: 
with  chemical  energy  as  source,  either  special  fuels  are 
required — in  the  candle,  kerosene  lamp — or  a  complex  gas 
plant.  Closing  the  switch  starts  the  motor,  whether  a 
small  fan  motor,  or  a  looo-horsepower  motor  supplying  the 
water  system  of  a  city  or  driving  the  railroad  train.  With 
fuel  as  source  of  energy,  boiler  plant,  steam  engine,  or 
turbine,  with  its  numerous  auxiliaries,  with  skilled  attend- 
ants, etc.,  are  necessary,  and  the  efficiency  is  low  except  in 
very  large  units.  To  appreciate  the  complexity  of  the 
conversion  of  the  chemical  energy  of  fuel,  compared  with 
the  simplicity  of  electrical  energy  conversion,  imagine  the 
domestic  fan  motor  with  coal  as  source  of  energy:  a  small 
steam  engine,  with  boiler  and  furnace,  attached  to  the  fan : 
to  start  the  fan,  we  have  to  make  a  coal  fire  and  raise 
steam  to  drive  the  engine.  This  illustrates  how  utterly 
unavailable  the  chemical  energy  of  fuel  is  for  general 
energy  distribution.  -General  energy  distribution,  there- 
fore, may  justly  be  said  to  date  from  the  introduction  of 
electric  power. 

Equally  true  is  the  reverse :  the  conversion  of  mechanical 
or  other  energy  into  electrical  is  simple  and  economical,  while 
the  conversion  into  chemical  energy  is  not.  Hence,  one 
of  the  two  large  sources  of  Nature's  energy,  the  water 
power,  was,  before  the  days  of  electrical  engineering, 
useless  except  to  a  very  limited  extent,  since  the  location 
of  the  water  power  is  rarely  such  that  the  energy  could  be 
used  at  its  source.  The  water  powers  thus  have  really 
been  made  available  only  by  the  development  of  electrical 
transmission. 

Characteristic  of  electrical  energy  is  that  it  can  be  concen- 
trated to  an  energy  density  higher  than  any  other  form  of 
energy,  and  results  can  thus  be  produced  by  it  which  no 


228  GENERAL  LECTURES 

other  form  of  energy  can  bring  about,  or  things  done 
directly  by  the  brute  force  of  energy,  as  we  may  say,  which 
formerly  had  to  be  brought  about  in  a  roundabout  way. 

Thus  iron  can  be  reduced  from  its  ores  by  the  chemical 
energy  of  coal  in  the  blast  furnace,  but  alu  minum  and  calcium 
cannot,  as  their  chemical  affinity  is  higher,  and  require  the 
higher  energy  concentration  available  with  electric  power. 
Iron  reduced  in  the  blast  furnace  combines  with  carbon 
to  cast  iron.  So  calcium  combines  with  carbon  in  the 
electric  furnace  to  carbide,  the  starting  material  of  acetylene, 
and  of  cyanamid  and  the  modern  fertilizer  industry. 
Platinum  can  just  be  melted,  and  quartz  softened,  in  the 
hottest  flames  of  combustion :  the  oxy-acetylene  flame  and 
the  oxy-hydrogen  flame.  But  in  the  electric  arc  platinum 
and  quartz  and  every  existing  substance,  even  tungsten  and 
carbon,  can  be  melted  and  distilled  or  sublimed.  Thus 
mighty  industries  have  grown  up  and  many  new  materials 
made  available  to  man,  as  aluminum,  silicon,  calcium, 
chromium,  the  carbides,  cyanamid,  acetylene,  etc. ;  others 
produced  in  a  cheaper  manner,  as  alkalies,  hypochlorites, 
phosphorus,  magnesium,  sodium,  etc. 

Electricity  as  such  is  the  most  useless  form  of  energy :  it  is 
not  found  in  Nature  in  industrially  available  quantities, 
and  finds  no  industrial  use  as  electrical  energy,  but  it  is 
always  produced  from  some  other  form  of  energy,  and  con- 
verted into  some  other  form  of  energy:  light,  mechanical 
energy,  chemical  energy,  heat,  etc.  That  is,  electrical 
energy  is  entirely  the  connecting  link,  the  intermediary, 
by  which  energy  is  brought  from  the  place  where  it  is 
found  to  the  place  where  it  is  used,  or  changed  from  the 
form  in  which  it  is  found  to  the  form  in  which  it  is  used. 
Thus,  on  first  sight,  it  appears  a  roundabout  way,  when,  for 
instance,  in  modern  electrical  ship  propulsion  an  electric 


ELECTRICAL  ENGINEERING  229 

generator  is  placed  on  the  steam  turbine,  a  motor  on  the 
ship  propeller,  a  few  feet  away,  though  it  is  not  different 
from  practically  every  other  use  of  electric  energy:  a  trans- 
mission link,  superior  to  any  other  transmission  by  the 
flexibility  given  by  the  simplicity  and  economy  of  conversion. 

The  most  serious  disadvantage  of  electrical  energy  is  that 
it  cannot  be  stored.  It  is  true,  there  exists  the  electric 
storage  battery,  and  it  is  used  to  a  large  extent  as  standby 
battery  in  high-grade  electric  distribution  systems  to  give 
absolute  reliability  of  service,  or  as  battery  floating  on  a 
railway  circuit  to  equalize  fluctuations  of  power,  or  in  special 
applications,  as  electric  automobiles.  It  does  not  really 
store  electrical  energy,  but  stores  energy  by  conversion  of 
the  electrical  into  chemical  energy,  and  reconversion,  in 
discharge,  of  the  chemical  into  electrical  energy. 

The  economic  efficiency  of  the  storage  battery — using  the 
term  in  the  broad  sense  including  interest  on  the  plant 
investment  and  depreciation — is  so  low  that  the  storage 
battery  does  not  come  into  consideration  in  the  industrial 
storage  of  energy — that  is,  in  making  the  rate  of  electrical 
energy  consumption  independent  of  that  of  energy  pro- 
duction. We  can  best  realize  this  by  comparing  electrical 
energy  with  the  chemical  energy  of  fuel:  the  latter  can  be 
stored  with  perfect  economy.  Thus,  when  using  fuel  as  the 
source  of  energy — in  a  steam  plant — no  serious  difficulty 
is  met  by  the  industry  even  if  the  fuel  supply  is  interrupted 
for  months,  as  in  the  case  of  a  supply  by  water,  through  the 
closing  of  the  navigation  by  ice :  we  would  simply  bring  in  a 
sufficient  coal  supply  to  last  until  the  navigation  opens  again 
in  spring.  But  with  electrical  energy  from  a  water  power 
we  could  never  dream  of  storing  energy  by  storage  battery 
to  last  over  the  2  or  3  months  during  which  the  river  runs 
dry  and  the  water  power  fails. 


230  GENERAL  LECTURES 

This  means  that  electrical  energy  must  be  consumed  at 
the  rate  at  which  it  is  produced,  and  the  cost  of  electrical 
energy  thereby  becomes  dependent  on  the  rate  of  the  en- 
ergy use.  This  is  not  the  case  with  most  other  forms  of 
energy,  as,  for  instance,  the  chemical  energy  of  fuel.  The 
price  of  a  ton  of  coal,  as  determined  by  the  cost  of  supply- 
ing it,  is  the  same  whether  I  dump  the  coal  into  a  furnace 
all  at  once,  or  whether  I  use  it  up  at  a  uniform  rate  in  a 
small  stove,  lasting  for  weeks.  If  I  consume  2400  cubic 
feet  of  gas  per  day,  its  cost  and  thereby  its  price  is  the  same 
whether  I  use  the  gas  at  a  uniform  rate  throughout  the 
day,  of  100  cubic  feet  per  hour,  or  whether  I  use  the  entire 
2400  cubic  feet  in  i  hour,  nothing  in  the  remaining  23 
hours :  the  gas  is  produced  at  whatever  rate  is  most  eco- 
nomical, stored  in  the  gas  holders  and  supplied  from  there 
at  whatever  rate  it  is  required  for  consumption.  If,  however, 
I  use  240  kilowatt-hours  of  electrical  energy  per  day,  it 
makes  a  very  great  difference  in  the  cost  of  supplying  this 
energy  whether  I  use  it  at  a  uniform  rate  of  10  kilowatt- 
hours  per  hour,  or  whether  I  use  the  entire  240  kilowatt- 
hours  in  i  hour,  nothing  in  the  remaining  23  hours.  In  the 
former  case,  10  kilowatts  of  generating  machinery  are 
necessary  in  the  steam  or  hydraulic  station  producing  the 
electric  energy,  10  kilowatts  capacity  in  transmission  lines, 
transformers,  substation  and  distribution  lines,  to  supply 
the  demand.  In  the  latter  case,  240  kilowatts  of  generating 
machinery,  240  kilowatts  of  line  and  transformer  capacity 
are  absorbed,  and  that  part  of  the  cost  of  supplying  the 
electric  energy,  which  consists  of  interest  in  investment  in 
the  plant,  of  depreciation,  etc. — in  short,  the  fixed  cost- 
is  24  times  as  high  in  the  latter  as  in  the  former  case. 
If  the  fixed  cost  approximates  half  the  total  cost  in  a  steam 
plant,  or  is  by  far  the  largest  part  of  the  total  cost  in  a 


ELECTRICAL  ENGINEERING  231 

hydraulic  plant,  it  follows  that  in  the  case  of  concentrated 
energy  used  during  a  short  time  the  cost  of  electric 
energy — and  with  it  the  price — will  be  very  much  larger- 
many  times,  possibly — than  in  the  case  of  a  uniform  energy 
consumption. 

Thus,  due  to  the  absence  of  storage,  the  cost  of  electrical 
energy  essentially  depends  on  the  uniformity  of  the  rate  of 
its  use — that  is,  on  the  load  factor,  as  the  ratio  of  the  average 
consumption  to  the  maximum  consumption. 

If  I  use  240  kilowatt-hours  of  electrical  energy  in  i  hour, 
nothing  during  the  remaining  23  hours,  that  part  of  the  cost 
which  is  the  fixed  cost  of  plant  investment  and  depreciation 
is  24  times  as  great  as  if  I  used  the  same  amount  of  energy  at 
a  uniform  rate  throughout  the  day.  In  the  former  case,  if 
somebody  else  uses  240  kilowatt-hours,  but  during  another 
hour  of  the  day,  the  same  plant  supplies  his  energy,  and 
the  fixed  cost  thus  is  cut  practically  in  two — that  is,  the  cost 
of  energy  to  both  of  us  is  materially  reduced.  Thus,  again, 
the  cost  of  electrical  energy,  and  with  it  its  price,  depends 
on  the  overlap  or  not  overlap  of  the  use  of  the  energy  by 
different  users,  the  so-called  "diversity  factor."  The 
greater  the  diversity  factor — that  is,  the  less  the  different 
uses  overlap  and  the  more  their  combination,  therefore, 
increases  the  uniformity  of  the  total  energy  demand,  the 
"station  load  factor"-— the  lower  is  the  energy  cost.  The 
cost  of  electrical  energy  for  lighting,  where  all  the  demand 
comes  during  the  same  part  of  the  day,  is  inherently  much 
higher  than  the  cost  for  uniform  24-hour  service  in  chemical 
works,  and  with  the  increasing  variety  of  load,  with  the 
combination  of  energy  supply  for  all  industrial  and  domestic 
purposes,  the  cost  of  energy  decreases. 

Thus,  unlike  other  forms  of  energy,  due  to  the  absence  of 
energy  storage,  electrical  energy  can  have  no  definite  cost 


232  GENERAL  LECTURES 

of  production,  but,  even  supplied  from  the  same  generating 
station,  its  cost  varies  over  a  wide  range,  depending  on  the 
load  factor  of  the  individual  use  and  the  diversity  factor 
of  the  different  uses. 

This  feature,  of  necessity,  must  dominate  the  economical 
use  of  electrical  energy  in  industrial,  domestic,  and  trans- 
portation service. 

II 

Civilization  results  in  the  complete  interdependence  of 
all  members  of  society  upon  each  other.  Amongst  the 
savages  each  individual,  family,  or  tribe  is  independent, 
produces  everything  it  requires.  In  the  barbarian  state 
some  barter  develops,  followed  by  trade  and  commerce 
with  increasing  civilization.  But  up  to  a  fair  state  of 
civilization — up  to  nearly  100  years  ago — all  necessities 
of  life  were  still  produced  in  the  immediate  neighbor- 
hood of  the  consumer,  each  group  or  territory  still  inde- 
pendent in  its  existence,  and  commerce  dealing  with  such 
things  only  which  were  not  absolutely  necessary  for  life. 
All  this  has  now  changed,  and  in  our  necessities  of  life,  as 
well  as  luxuries,  we  depend  on  a  supply  from  distances  of 
hundreds  and  thousands  of  miles:  the  whole  world  con- 
tributes in  the  supply  of  our  food,  clothing,  building  ma- 
terials, etc. 

That  means,  our  existence  is  dependent  on  an  efficient 
and  reliable  system  of  transportation  and  distribution  of  all 
needs  of  civilized  life.  Such  has  been  developed  during  the 
last  century  in  the  system  of  steam  railroads,  which,  in 
taking  care  of  the  transportation  and  distribution  of  com- 
modities, have  made  modern  civilization  possible.  For 
civilization  means  separation  of  production,  in  time  and  in 
location,  from  consumption,  to  secure  maximum  economy. 


ELECTRICAL  ENGINEERING  233 

The  necessities  of  civilized  life  consist  of  two  groups :  ma- 
terials and  energy.  'Our  transportation  system  takes  care 
of  materials,  but  cannot  deal  with  the  supply  of  energy,  and 
the  failure  of  an  efficient  energy  supply  has  been  and  still 
is  the  most  serious  handicap  which  retards  the  advance  of 
civilization.  The  transportation  system  could  deal  with 
the  energy  supply  only  in  an  indirect  manner,  by  the  supply 
of  materials  as  carriers  of  energy,  and  when  our  railroads 
carry  coal  it  is  not  the  material  which  we  need,  but  the 
energy  which  it  carries.  But  this  energy  is  available  only 
to  a  very  limited  extent,  as  heat,  and  as  mechanical  power 
in  big  steam  units ;  most  of  the  energy  demands  of  civilized 
life  could  not  be  satisfied  by  it.  In  any  country  village 
far  away  from  the  centers  of  civilization  we  have  no  diffi- 
culty to  have  delivered  to  us  any  material  produced  any- 
where in  the  world;  but  even  in  the  centers  of  civilization 
we  could  not  get  the  energy  to  run  a  sewing  machine  or 
drive  a  fan  without  electric  power.  Thus,  just  as  our  steam 
railways  and  express  companies  take  care  of  the  transporta- 
tion and  distribution  of  materials,  so  civilization  requires 
a  system  of  transmission  and  distribution  of  energy,  and 
our  electric  circuits  are  beginning  to  do  this;  and  just  as  50 
to  75  years  ago  in  the  steam  railroads,  steamship  lines,  etc., 
the  system  of "  transportation  and  distribution  of  materials 
was  developed,  so  we  see  all  around  us  in  the  electric 
transmission  systems  the  development  of  the  system  of  the 
world's  energy  transmission  in  progress  of  development. 
When  we  see  local  electric  distribution  systems  combining, 
the  big  electric  systems  of  our  capital  cities  reaching  out 
over  the  country,  transmission  lines  interconnecting  to 
networks  covering  many  thousands  of  square  miles,  this 
is  not  merely  the  result  of  the  higher  economy  of  coopera- 
tion, of  mass  production,  but  it  is  the  same  process  which 


234  GENERAL  LECTURES 

took  place  in  the  steam  railroad  world  some  time  ago,  as  a 
necessary  requirement  of  coordination  to  carry  out  their 
function  as  carriers  and  distributers  of  materials  in  the 
case  of  the  railroads,  of  energy  in  the  case  of  the  electric 
systems. 

We  must  realize  this  progress,  and  the  forces  which  lead 
to  it,  so  as  to  understand  what  is  going  on,  and  to  assist 
in  the  proper  development,  in  avoiding,  in  the  creation  of 
the  country's  electrical  network,  whatever  mistakes  have 
been  made  in  the  development  of  the  country's  railway 
network. 

Electricity,  thus,  is  taking  over  the  energy  supply  re- 
quired by  civilization  as  the  only  form  of  energy  which,  by 
its  simplicity  and  economy  of  conversion,  combined  with 
economical  transmission,  is  capable  of  supplying  all  the 
energy  demands,  from  the  smallest  domestic  need  to  the 
biggest  powers.  As  we  now  begin  to  realize,  the  economic 
function  of  the  steam  engine  is  not  the  energy  supply  at  the 
place  of  consumption,  from  the  chemical  energy  of  coal — it 
is  too  complicated  and  inefficient  for  this — but  it  is  the 
conversion  of  chemical  energy  of  coal  into  electrical  energy 
in  bulk,  for  transmission  and  distribution  to  the  places  of 
consumption. 

If,  then,  electric  power  takes  the  place  of  steam  power  in 
our  industries,,  etc.,  it  is  not  merely  the  substitution  of  the 
electric  motor  for  the  steam  engine  or  turbine.  Such 
would  rarely  realize  the  best  economy.  The  method  of 
operation  in  all  our  industries,  and  especially  those  requiring 
considerable  power,  is  largely — more  than  usually  realized 
— determined  by  the  characteristics  of  the  power  supply, 
and  what  is  the  most  economical  method  with  the  steam 
engine  as  source  of  power  may  be  very  uneconomical  with 
electric  power  supply,  and  electric  power  supply  often 


ELECTRICAL  ENGINEERING  235 

permits,  a  far  more  economical  method  of  operation  which 
was  impossible  with  steam  power.  Thus  the  introduction 
of  electricity  as  the  medium  of  distributing  the  world's 
energy  demand  means  a  reorganization  of  our  industrial 
methods,  to  adapt  the  same  to  the  new  form  of  power. 

For  instance,  the  steam  engine  requires  skilled  attendance, 
and  with  its  boiler  plant,  auxiliaries,  etc.,  is  a  complex 
apparatus,  is  economical  only  in  large  units.  Thus,  when 
operating  a  factory  or  mill  by  steam  power,  one  large  engine 
is  used,  driving  by  shafts  and  countershafts,  by  pulleys 
and  belts,  and  possibly  wasting  half  or  more  of  its  energy 
in  the  mechanical  transmission  to  the  driven  machines. 
But  we  could  not  economically  place  a  steam  engine  at 
every  one  of  the  hundreds  of  machines  in  the  factory. 
Substituting  electrical  power  by  replacing  the  engine  by 
one  large  electric  motor  would  be  very  uneconomical,  as  we 
can  place  a  motor  at  every  driven  machine,  and  these  small 
motors  are  practically  as  efficient — within  very  few  per 
cent. — as  one  big  motor  would  be,  and  all  the  belting  and 
shafting,  with  its  waste  of  energy,  inconvenience,  and 
danger,  vanishes.  With  the  steam  engine  as  source  of 
power,  to  run  one  or  two  machines  only,  to  complete  some 
work,  requires  keeping  the  big  engine  in  operation,  and 
therefore  is  extremely  wasteful.  With  individual  electric 
motors  the  economy  is  practically  the  same,  whether  only 
one  or  two  motors  are  used,  or  the  entire  factory  is  in 
operation.  On  the  other  hand,  with  the  steam  engine,  it 
makes  no  difference  in  the  cost  of  power  whether  it  is  in 
operation  from  8  a.m.  to  6  p.m.,  or  from  6  a.m.  to  4  p.m. 
With  electric  power,  in  the  former  case  the  power  demand 
would  overlap  with  whatever  lighting  load  the  same  supply 
circuit  carries,  but  would  not  in  the  latter  case,  and  the 
latter  case  thus  would  give  a  better  load  factor  of  the  electric 


236  GENERAL  LECTURES 

circuit,  and  thereby  a  lower  cost  of  power.  Again,  with 
electric  power,  if  very  large  power  demands  could  be  re- 
stricted to  the  periods  of  light  load  on  the  electric  supply 
systems,  this  would  reduce  the  cost  of  power.  Nothing 
like  this  exists  with  the  steam  engine. 

Electrical  energy  thus  makes  the  power  users  econom- 
ically more  dependent  upon  each  other,  and  thereby  exerts 
a  strong  force  toward  industrial  coordination — that  is, 
cooperation. 

Another  illustration  of  the  industrial  reorganization  re- 
quired to  derive  the  full  benefit  of  electric  power  is  afforded 
by  the  traction  problem.  Very  often  a  study  of  the  electri- 
fication of  a  railway  shows  no  economical  advantage  in  the 
replacement  of  the  steam  locomotive  by  the  electric  locomo- 
tive, even  when  considering  only  passenger  service.  At 
the  same  time,  an  electric  railway  may  parallel  the  same 
steam  railway,  offer  better  service  at  lower  price,  and  show 
financially  better  returns  than  the  steam  railway.  But 
so,  also,  in  the  early  days  of  steam,  the  steam  engine  in 
place  of  the  horse  in  front  of  the  stage  coach  was  no  success, 
and  still  the  stage  coach  has  gone  and  the  steam  locomotive 
has  conquered;  but  it  did  not  by  replacing  the  horse,  but 
by  developing  a  system  suited  to  the  characteristics  of  the 
steam  engine.  The  same  repeats  now  in  the  relation  of 
steam  traction  and  electric  traction.  The  steam  engine  is 
most  economical  in  the  largest  units,  and  the  economy  of 
steam  railway  operation  depends  on  the  concentration  of 
the  load  in  as  few  and  as  large  units  as  possible :  therefore, 
the  largest  locomotive  which  can  pass  through  bridges  and 
around  curves.  Exactly  the  reverse  is  the  condition  of 
economy  of  electric  traction:  the  economy  depends  on  the 
distribution  of  the  load  as  uniformly  as  possible  in  space 
and  in  time — that  is,  small  units  at  frequent  intervals — 


ELECTRICAL  ENGINEERING  237 

and  therefore,  while  steam  traction  has  gone  to  larger  and 
larger  units,  in  electric  traction  even  the  trailer  car,  so 
frequently  used  in  the  early  days,  has  practically  vanished. 
Obviously,  then,  the  electric  motor  cannot  economically 
compete  with  the  steam  engine  under  the  conditions  of 
maximum  economy  of  steam  and  minimum  economy  of 
electric  operation,  and  electric  traction  under  steam  trac- 
tion conditions  shows  marked  economy  only  in  the  case  of 
such  heavy  service  that  the  maximum  permissible  train 
units  follow  each  other  at  the  shortest  possible  intervals— 
that  is,  give  maximum  uniformity  of  load — and  thus  the 
economic  requirements  of  both  forms  of  power  coincide. 
These  two  instances  may  illustrate  the  changes  in  industrial 
operation  which  the  introduction  of  electric  power  requires 
and  which  are  taking  place  today. 

To  conclude,  then :  Electric  energy  is  the  only  form  which 
is  economically  suited  for  general  energy  transmission  and 
distribution.  Civilization  depends  on  the  supply  of  mate- 
rials and  of  energy  as  its  two  necessities.  The  supply  of 
materials  is  taken  care  of  by  the  transportation  system  of 
the  world.  The  supply  of  energy  is  being  developed  by 
the  electrical  transmission  system,  which  with  regard  to 
energy  becomes  what  the  railway  system  is  with  regard  to 
materials.  Introduction  of  electric  power  in  place  of  other 
forms  of  power  rarely  can  be  a  mere  substitution,  but 
usually  requires  a  change  of  the  methods  of  power  applica- 
tion, a  reorganization  of  the  industry,  to  secure  maximum 
economy. 

Before  our  eyes  we  thus  see  today  taking  place  the  organi- 
zation of  the  universal  energy  supply  of  the  civilized  world, 
by  electric  transmission  and  distribution,  and  while  the 
details  of  the  structure  are  still  changing  and  undeveloped, 
we  can  already  see  the  general  outlines  and  methods,  the 


238  GENERAL  LECTURES 

general  principles  of  the  organization  of  the  generating 
systems,  the  transmission  and  distribution  circuits  and  their 
interconnection . 

The  huge  metropolitan  steam-turbine  stations,  of  hun- 
dreds of  megawatts  capacity,  usually  are  the  centers  of 
electric  power  generation :  often  several  such  stations  in 
the  same  city,  as  Chicago,  New  York,  etc.,  tied  together 
into  one  unit. 

Smaller  steam-turbine  stations  at  strategic  points  through- 
out the  country  interconnect  with  the  metropolitan  system 
and  give  control  and  steadiness  to  the  power. 

Where  large  water  powers  exist,  hydraulic  stations  feed 
into  the  system,  and  induction  generator  stations,  auto- 
matic and  without  attention,  are  just  beginning  to  make 
their  appearance  in  collecting  smaller  water  powers  and 
feeding  into  the  synchronous  stations  as  receivers. 

In  some  territories,  as  Chicago,  New  York,  practically 
all  generation  may  be  by  steam  turbine,  in  other  hydraulic 
power  may  preponderate,  as  in  the  South  and  the  far  West, 
or  steam  and  water  power  may  share  in  the  supply,  as  in 
the  upper  Hudson,  the  Niagara,  the  New  England  develop- 
ments. 

From  the  generating  stations  issue  the  power  distribution 
lines,  supplying  power  in  bulk  to  substations  or  to  large 
industries,  as  railway  systems,  cities,  mills  and  factories. 
In  densely  populated  districts,  as  large  cities,  generating 
stations  as  well  as  power  distribution  lines  usually  are  about 
10,000  volts,  the  latter  underground  cable;  throughout 
the  country,  33,000  seems  to  come  into  favor  as  the  most 
convenient  voltage  for  power  distribution. 

Trunk  lines  then  interconnect  the  generating  stations 
and  centers  of  electrical  development,  water  powers,  cities, 
industrial  territories,  at  voltages  of  100  to  200  kilovolts. 


ELECTRICAL  ENGINEERING  239 

On  the  other  side,  from  the  substations  supplied  by 
the  power  distribution  circuits,  issue  for  general  dis- 
tribution the  primary  distribution  lines,  almost  always 
2300  volts,  or  four- wire  2300  volts,  that  is,  4000  volts 
between  the  phases,  and  from  step-down  transformers  fed 
by  the  primary  lines  then  issue  the  secondary  lines,  three- 
wire  no  volts,  usually  single-phase. 

We  thus  see  the  development  of  four  systems  of  circuits 
superimposed  upon  each  other: 

The  three- wire  no-volt  secondary  distribution,  for 
lighting  and  general  domestic  use,  including  smaller  motors. 

The  23oo-volt  primary  distribution,  single-phase  wire  for 
feeding  secondary  circuits,  three-phase  for  industrial  power, 
etc. 

The  io,ooo-volt  underground  cable  or  33,ooo-volt  over- 
head power  distribution,  supplying  the  substations  from 
which  radiate  the  primary  distribution  circuits;  supplying 
also  other  distribution  substations,  as  converter  substation 
for  6oo-volt  direct-current  railway  or  three- wire  i25-volt 
direct-current  general  distribution;  supplying  also  large 
factories,  mills,  etc. 

The  trunk  lines  of  very  high  voltage,  interconnecting 
the  power  distribution  and  generating  systems. 

The  power  distribution  circuits,  especially  the  overhead 
33,ooo-volt  ones,  less  the  underground  cable  systems, 
frequently  are  interconnected  networks,  carrying  heavy 
power.  So  also  are  the  high- voltage  trunk  lines  networks, 
of  much  larger  mesh,  however,  but  often  carrying  less 
power,  rather  used  to  exchange  power  between  systems, 
with  the  varying  demands  of  the  systems. 

Primary  and  secondary  distribution,  however,  commonly 
is  radial,  that  is,  the  feeders  are  not  interconnected,  but 
separately  controlled  from  the  stations. 


240  GENERAL  LECTURES 

As  seen,  small  distributed  power  and  light  is  supplied 
from  the  secondary  distribution;  larger  motors,  small 
factories,  etc.,  from  the  primary  distribution,  while  large 
users  of  powers,  as  railways,  cities  and  villages,  industrial 
establishments,  are  served  from  the  power  distribution  cir- 
cuits, while  the  trunk  lines  serve  to  unify  the  entire  system. 


APPENDIX  II 


OVERHEAD  LINE  TABLES 

Per  mile  of  single  wire  of  three-phase  or  single-phase  line.     Solid  round 

copper  wire 


1* 

6 

OOO            O     M     N            f»5't«O          O     f^  00            O>    O 

6o-cycle  reactance,  x«o 

Between  wires,  in  ohms 

1  6  feet 

Tj-oon           CO    >O  O           t^OvO           M    i*5    ^         vOt^oO           Oi 
f*5<*5^t         ^t^t^l-         ^t  •'t  >o         10  to  10         101010        OO 

1 

oo 

ooc?O          r-OiO          Sro^j-        o'S-oo          O«f«5         -<t  m 

.      .      .                  .      .            .      .      .            .      . 

1 

MOO            MrO-3-          Ot^OO            O«<"O           Tior^          OOO 

ooo        odd        odd        ooo        ooo        oo 

1 

M 

OOrj-         ot-oo          0*2°           -d^t^         00011           NTt 

.     .     .         .     .     .         .     .     .         .     .     .         .     .     .         .     . 

25-cycle  reactance,  xzt 

W 

s 

jr 

o 
c 

I 

c 

1 

0) 

PQ 

1 

IONOO            fJOi't           OOw            t^f*500            TfO>O           wt^ 
rfOt^          OOOOOi           OOM            ii<SN            roi-rj            IO1O 

OOO           OOO           OOO           OOO           OOO           OO 

1 

00 

3£S     8^:     Jr^'S     SS-S     ZZ2    $Z 

doo        doo        odd        ooo        ooo        oo 

1 

•t 

1 

t^Ovn              MNM              rOTTTf             1O1OO             Ot~-00             OOOi 

odd        odd        odd        odd        odd        do 

-SlS3"fc[ 

I 

iN-cf          OOO           OOO           OOO           OOO           OO 
000           OOO           OOO           MUM           NNCO          •*   m 

SJIUI 
Ut    'B3Jy 

r8 
ij 

OOO           000           OOO           OOO           OOO           OO 
000            OOOO            O-Ot-           f)tO            ION"            000 

OOO            nr^PO           lOfOO            N*iCO          OOO            **5O 
§O"~.           MOfO          OOOO           lOTj-ro          NNM           MH 
IO     O4                 Ci      1      h-l                 11 

M 

J3}3tuBiQ 

1 

OOO          OnO           (Nooio           <NOOC          O^tN           iO 

- 

^t{3l3^ 

Pounds 

§OO           OOiO          O«OO           OOO           OtOtO          OO 
OO          OOOOM           OifCO           rft^ro           Nf»5O           «O 

3-tIM. 

jo  SZKJ 

i 

•      •      •           OOO           OM«           POtiO         Ot-00           OiO 

16 


241 


242  GENERAL  LECTURES 

In  stranded  copper  cable  the  2  5 -cycle  reactance  is  about 
0.007  onm  IGSS>  the  6o-cycle  reactance  0.017  ohm  less  than 
in  solid  round  wire  of  same  resistance. 

In  stranded  aluminum  cable  the  2  5 -cycle  reactance  is 
0.02  ohm  less,  the  6o-cycle  reactance  about  0.048  ohm  less 
than  in  solid  round  copper  wire  of  same  resistance. 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY 

Return  to  desk  from  which  borrowed. 
Th|  book  js  DUEMr die  last  da^e  Damped  below. 


AUG  1 4  1950 
JUN 


OCT6 


INTER-LlBRARY 
LOAN 


LD  21-100m-9,'48(B399sl6)476 


Engine«ring 
Lflwary 


VERSITY  OF   CALIFORNIA 
DEPARTMENT    OF   CIVIL    ENGINEERING 

BERKELEY,  CALIFORNIA 
UNIVERSITY  OF  CALIFORNIA  LIBRARY 


